mbcache: don't BUG() if entry cache cannot be allocated

mbcache can be a module that is loaded long after startup, when someone
asks to mount an ext2 or ext4 filesystem.  Therefore it should not BUG()
if kmem_cache_create() fails, but rather just fail the module load.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>