zdb_il: use flex array member to access ZIL records

[robn]

Motivation and Context

Working on something else, I had cause to compile with optimisations disabled, and the build broke.

$ ./configure ... CFLAGS='-g -O0' CXXFLAGS='-g -O0'
...
$ make
...
cmd/zdb/zdb_il.c: In function ‘zil_prt_rec_create’:
cmd/zdb/zdb_il.c:82:31: error: ‘strlen’ reading 1 or more bytes from a region of size 0 [-Werror=stringop-overread]
   82 |                 link = name + strlen(name) + 1;
      |                               ^~~~~~~~~~~~
In file included from cmd/zdb/zdb_il.c:43:
./include/sys/zil.h:271:25: note: at offset 104 into source object ‘lr_create’ of size 104
  271 |         _lr_create_t    lr_create;      /* common create portion */
      |                         ^~~~~~~~~
./include/sys/zil.h:271:25: note: at offset [168, 17179869452] into source object ‘lr_create’ of size 104
cmd/zdb/zdb_il.c: In function ‘zil_prt_rec_rename’:
cmd/zdb/zdb_il.c:128:27: error: ‘strlen’ reading 1 or more bytes from a region of size 0 [-Werror=stringop-overread]
  128 |         char *tnm = snm + strlen(snm) + 1;
      |                           ^~~~~~~~~~~
./include/sys/zil.h:333:25: note: at offset 48 into source object ‘lr_rename’ of size 48
  333 |         _lr_rename_t    lr_rename;      /* common rename portion */
      |                         ^~~~~~~~~
...

I suppose in an optimised build, these things are gone long before anything can see them and complain them.

Description

In 6f50f8e we added flex arrays to lr_XX_t structs to silence kernel bounds check warnings. Userspace code was mostly not updated to use them though.

This commit switches the handful of complaints over to use the flex arrays, and all is well again.

How Has This Been Tested?

Compiled with and without optimisations, and with/without ASAN. No complaints from anyone.

Ran the slog test suite, a major user of zdb -i. All passed.

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.