zsmalloc: preemptible object mapping

This is Part II of the series [1] that makes zram read() and write()
preemptible.  This part focuses only zsmalloc because zsmalloc imposes
atomicity restrictions on its users.  One notable example is object
mapping API, which returns with:
a) local CPU lock held
b) zspage rwlock held

First, zsmalloc is converted to use sleepable RW-"lock" (it's atomic_t
in fact) for zspage migration protection.  Second, a new handle mapping
is introduced which doesn't use per-CPU buffers (and hence no local CPU
lock), does fewer memcpy() calls, but requires users to provide a
pointer to temp buffer for object copy-in (when needed).  Third, zram is
converted to the new zsmalloc mapping API and thus zram read() becomes
preemptible.

[1] https://lore.kernel.org/linux-mm/20250127072932.1289973-1...

RFC -> v1:
- Only zspage->lock (leaf-lock for zs_map_object()) is converted
  to a preemptible lock.  The rest of the zspool locks remain the
  same (Yosry hated with passion the fact that in RFC series all
  zspool looks would become preemptible).
- New zs object mapping API (Yosry hated RFC API with passion).
  We know have obj_read_begin()/obj_read_end() and obj_write().
- obj_write() saves extra memcpy() calls for objects that span two
  physical pages.
- Dropped zram deferred slot-free-notification handling (I hated
  it with passion)

Sergey Senozhatsky (6):
  zsmalloc: factor out pool locking helpers
  zsmalloc: factor out size-class locking helpers
  zsmalloc: make zspage lock preemptible
  zsmalloc: introduce new object mapping API
  zram: switch to new zsmalloc object mapping API
  zram: add might_sleep to zcomp API

 drivers/block/zram/zcomp.c    |   6 +-
 drivers/block/zram/zcomp.h    |   2 +
 drivers/block/zram/zram_drv.c |  28 +--
 include/linux/zsmalloc.h      |   8 +
 mm/zsmalloc.c                 | 372 ++++++++++++++++++++++++++--------
 5 files changed, 311 insertions(+), 105 deletions(-)

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2.48.1.262.g85cc9f2d1e-goog