sysctl: Generalize proc handler converter creation

Implement new converter generation macros that remove repeated logic and
prep the proc handler converter infrastructure for exposure to the
greater kernel. Macros will be exposed in a later series so they can be
used in jiffies.c and pipe.c (like in this branch [1]).

What is done?
=============

Three integer macros (SYSCTL_{KERN_TO_USER,USER_TO_KERN}_INT_CONV and
SYSCTL_INT_CONV_CUSTOM) are created.
  * SYSCTL_INT_CONV_CUSTOM: creates a bi-directional (handles both user
    to kernel and kernel to user writes) converter that optionally
    implements a range checker for when kernel memory is written.
  * SYSCTL_KERN_TO_USER_INT_CONV: is a uni-directional converter that
    writes to a user buffer avoiding tears with READ_ONCE and setting
    the negp variable appropriately; it generates functions that do not
    fail.
  * SYSCTL_USER_TO_KERN_INT_CONV: is a uni-directional converter that
    writes to a kernel buffer, checks for integer overflow and avoids
    tears by using with WRITE_ONCE; returns -EINVAL when an overflow is
    detected.

Two unsigned integer macros (SYSCTL_USER_TO_KERN_UINT_CONV and
SYSCTL_UINT_CONV_CUSTOM) are created.
  * SYSCTL_UINT_CONV_CUSTOM: Same as SYSCTL_INT_CONV_CUSTOM except that
    there are no special cases for negative values.
  * SYSCTL_USER_TO_KERN_UINT_CONV: Same as SYSCTL_USER_TO_KERN_INT_CONV
    except that there is no need to indicate when the value is negative.
    The check for overflow is done against UINT_MAX instead of INT_MAX.

For now these macros produce static functions that are used from within
kernel/sysctl.c. The idea is to move them to include/kernel/sysctl.h in
another series so they can be used to create custom converters.

Why it is done?
===============

Motivation is to move non-sysctl logic out of kernel/sysctl.c which had
become a dumping ground for ctl_tables until this trend was changed by
the commits leading to (and including) 73184c8e4ff4 ("sysctl: rename
kern_table -> sysctl_subsys_table"). This series does not move the
jiffie logic out, but it sets things up so it can eventually be evicted
from kernel/sysctl.c.

Testing
=======

* I ran this through the sysctl selftests and sysctl kunit tests on an
  x86_64 arch
* This also goes through the sysctl-testing 0-day CI infra.

Versions
========

Changes in v2:
- Corrected cover letter wording
- Added macros for unsigned int converters. Three new commits:
    - sysctl: Create macro for user-to-kernel uint converter
    - sysctl: Add optional range checking to SYSCTL_UINT_CONV_CUSTOM
    - sysctl: Create unsigned int converter using new macro
  Added to prepare for when the macros will be used from outside of
  sysctl.c (to be added in another series)
- Link to v1: https://lore.kernel.org/r/20251013-jag-sysctl_conv-v1-0-4...

Any comments are greatly appreciated

[1] https://git.kernel.org/pub/scm/linux/kernel/git/joel.gran...

---
Joel Granados (11):
      sysctl: Replace void pointer with const pointer to ctl_table
      sysctl: Remove superfluous tbl_data param from "dovec" functions
      sysctl: Remove superfluous __do_proc_* indirection
      sysctl: Indicate the direction of operation with macro names
      sysctl: Discriminate between kernel and user converter params
      sysctl: Create converter functions with two new macros
      sysctl: Create integer converters with one macro
      sysctl: Add optional range checking to SYSCTL_INT_CONV_CUSTOM
      sysctl: Create unsigned int converter using new macro
      sysctl: Add optional range checking to SYSCTL_UINT_CONV_CUSTOM
      sysctl: Create macro for user-to-kernel uint converter

 fs/pipe.c              |   6 +-
 include/linux/sysctl.h |   5 +-
 kernel/sysctl.c        | 694 +++++++++++++++++++++----------------------------
 3 files changed, 298 insertions(+), 407 deletions(-)
---
base-commit: e5f0a698b34ed76002dc5cff3804a61c80233a7a
change-id: 20251012-jag-sysctl_conv-570844f5fdaf

Best regards,
-- 
Joel Granados <joel.granados@kernel.org>