Linux 6.12: avoid kmem_cache_create redefinition

torvalds/linux@b2e7456b5c25 makes kmem_cache_create() a macro, which
gets in the way of our our own redefinition, so we undef the macro first
for our own clients. This follows what we did for kmem_cache_alloc(),
see e951dba48.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16582