lkdtm: use SLAB_NO_MERGE instead of an empty constructor

lkdtm: use SLAB_NO_MERGE instead of an empty constructor

Use SLAB_NO_MERGE flag to prevent merging instead of providing an
empty constructor. Using an empty constructor in this manner is an abuse
of slab interface.

The SLAB_NO_MERGE flag should be used with caution, but in this case,
it is acceptable as the cache is intended soley for debugging purposes.

No functional changes intended.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://lore.kernel.org/r/20250318014533.1624852-1-harry.yoo@oracle.com
Signed-off-by: Kees Cook <kees@kernel.org>