zfs_log: add flex array fields to log record structs

[robn]

Motivation and Context

Silence Linux warnings since ~6.11.

Closes #16501.

Description

ZIL log record structs (lr_XX_t) are frequently allocated with extra space after the struct to carry variable-sized "payload" items.

Linux 6.10+ compiled with CONFIG_FORTIFY_SOURCE has been doing runtime bounds checking on memcpy() calls. Because these types had no indicator that they might use more space than their simple definition, __fortify_memcpy_chk will frequently complain about overruns eg:

memcpy: detected field-spanning write (size 7) of single field "lr + 1" at /home/robn/zfs/module/zfs/zfs_log.c:425 (size 0)
memcpy: detected field-spanning write (size 9) of single field "(char *)(lr + 1)" at /home/robn/zfs/module/zfs/zfs_log.c:593 (size 0)
memcpy: detected field-spanning write (size 4) of single field "(char *)(lr + 1) + snamesize" at /home/robn/zfs/module/zfs/zfs_log.c:594 (size 0)
memcpy: detected field-spanning write (size 7) of single field "lr + 1" at /home/robn/zfs/module/zfs/zfs_log.c:425 (size 0)
memcpy: detected field-spanning write (size 9) of single field "(char *)(lr + 1)" at /home/robn/zfs/module/zfs/zfs_log.c:593 (size 0)
memcpy: detected field-spanning write (size 4) of single field "(char *)(lr + 1) + snamesize" at /home/robn/zfs/module/zfs/zfs_log.c:594 (size 0)
memcpy: detected field-spanning write (size 7) of single field "lr + 1" at /home/robn/zfs/module/zfs/zfs_log.c:425 (size 0)
memcpy: detected field-spanning write (size 9) of single field "(char *)(lr + 1)" at /home/robn/zfs/module/zfs/zfs_log.c:593 (size 0)
memcpy: detected field-spanning write (size 4) of single field "(char *)(lr + 1) + snamesize" at /home/robn/zfs/module/zfs/zfs_log.c:594 (size 0)

To fix this, this commit adds flex array fields to all lr_XX_t structs that require them, and then uses those fields to access that end-of-struct area rather than more complicated casts and pointer addition.

I'll note here that I wrote this a few weeks ago and kind of forgot about it. I remember I didn't love it; it felt messy in a couple of places. But, it passes tests, and 6.11 is here and 2.3 is looming, so its probably better to post this sooner rather than later.

How Has This Been Tested?

Full ZTS run completed against 6.11.0.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[darkbasic]

Could you please backport this into the 2.2.x branch? It's been two months without any commit on staging: is 2.3 nearing release or what?

[darkbasic]

Yeah it looks like so: https://github.com/openzfs/zfs/releases/tag/zfs-2.3.0-rc1