timekeeping/ntp: Fix drift tracking precision

This is the bugfix subset of the RFC series last posted at
https://lore.kernel.org/all/20260520135207.37826-1-dwmw2@...

These patches stand alone and fix several long-standing precision issues 
in the kernel's NTP drift tracking. The feed-forward clock discipline
which was the reason I was *looking*, is dropped for now.

Patch 1: MAINTAINERS update
  Add Miroslav Lichvar as timekeeping reviewer.

Patches 2-3: Timekeeping bugfixes
  2. Remove stale xtime_remainder from ntp_error accumulation.
  3. Account for monotonicity adjustment in ntp_error.

  These fix systematic drift in ntp_error tracking that caused the
  timekeeping dithering to fight against NTP corrections rather than
  assist them.

Patch 4: Independent bugfix
  4. Guard against divide-by-zero during clocksource recalibration.

  Prevents an oops on KVM guests with clocksource=tsc when the TSC
  frequency is recalibrated during boot.

Patches 5-7: NTP rework — eliminate tick_length_base
  5. Drive time_offset skew via per-tick ntp_error transfer instead of
     tick_length inflation, with mult adjustment for dithering bandwidth.
  6. Convert adjtime() to use time_offset directly instead of inflating
     tick_length, removing the rounding loss that prevented convergence.
  7. Remove tick_length_base entirely — tick_length is now always the
     NTP-disciplined value with no per-tick inflation.

  This eliminates the dual-accounting between tick_length and
  tick_length_base that was a source of rounding errors and made the
  code harder to reason about.

The net effect is cleaner accounting in the timekeeping code: the
dual tick_length/tick_length_base bookkeeping is eliminated, and the
ntp_error tracking no longer accumulates systematic bias from stale
remainders or unaccounted monotonicity adjustments.

I've tested this with added debugging tracking the absolute values of
the three time references:
 • (A): xtime, reported as CLOCK_REALTIME.
 • (B): xtime+ntp_error, the time NTP *wants* to report right now.
 • (C): xtime+ntp_error+time_offset, the time NTP wants to skew to.

With the fixes, the ntp_error and time_offset deltas remain entirely
consistent and the kernel perfectly skews to and tracks the reference
setting that it's asked to (although the actual *setting* is now
absent from this series; it's just using the existing tick_length
and time_offset parameters).

Changes since RFC v3:
 - Dropped patches 8-10 (feed-forward infrastructure, ptp_vmclock
   integration, vmclock_host) — those will be reposted separately
   once Thomas's timekeeping branch lands.
 - No code changes to patches 1-7 vs the RFC.

Changes since RFC v2:
 • Renamed "clawback" to "monotonicity adjustment" throughout (patch 2).
 • Drop the exponential tail clamping (v2 patch 3).
 • Convert adjtime() to use time_offset to deliver skew too, and remove
   the separate 'tick_length_base' as adjusting tick_length directly is
   no longer used to skew the clock. The skew_delta basically does the
   same thing, but is easier to get the accounting right.
 • The timekeeping_set_reference() API (patch 8) now takes tk_core.lock
   and computes the phase offset internally, eliminating the race window
   that existed in v2 between setting the reference and the tick code
   consuming it.
 • vmclock_host (patch 10) is no longer marked WIP — it has a selftest
   and proper locking (but is still RFC).
 • Added MAINTAINERS entry for Miroslav Lichvar as timekeeping reviewer
   (patch 1).

David Woodhouse (7):
      MAINTAINERS: Add Miroslav as timekeeping reviewer
      timekeeping: Remove xtime_remainder from ntp_error accumulation
      timekeeping: Account for monotonicity adjustment in ntp_error
      timekeeping: Guard against divide-by-zero in timekeeping_adjust
      timekeeping: Drive time_offset skew via per-tick ntp_error transfer
      ntp: Convert adjtime() to use time_offset instead of tick_length inflation
      ntp: Remove tick_length_base, use tick_length directly

 MAINTAINERS                         |  1 +
 include/linux/timekeeper_internal.h |  4 +-
 kernel/time/ntp.c                   | 99 ++++++++++++++++++++++++++-----------
 kernel/time/ntp_internal.h          |  3 ++
 kernel/time/timekeeping.c           | 41 ++++++++++++---