kmem_cache_destroy() can handle NULL pointer correctly, so there is no need
to check NULL pointer before calling kmem_cache_destroy().
For kernel versions before v4.3 there is a backport of
kmem_cache_destroy() that checks for a null pointer.
This patch fixes the following checkpatch warnings:
WARNING:NEEDLESS_IF: kmem_cache_destroy(NULL) is safe and this check
is probably not required