dlm: NULL check before kmem_cache_destroy is not needed

kmem_cache_destroy(NULL) is safe, so removes NULL check before
freeing the mem. This patch also fix ifnullfree.cocci warnings.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Signed-off-by: David Teigland <teigland@redhat.com>