mbcache: make sure c_entry_count is not decremented past zero

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
CC: Eric Biggers <ebiggers@google.com>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Jan Kara <jack@suse.cz>