mbcache: automatically delete entries from cache on freeing

[ Upstream commit 307af6c8 ]

Use the fact that entries with elevated refcount are not removed from
the hash and just move removal of the entry from the hash to the entry
freeing time. When doing this we also change the generic code to hold
one reference to the cache entry, not two of them, which makes code
somewhat more obvious.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-10-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Stable-dep-of: a44e84a9 ("ext4: fix deadlock due to mbcache entry corruption")
Signed-off-by: Sasha Levin <sashal@kernel.org>