ext2: avoid deleting xattr block that is being reused

ext2: avoid deleting xattr block that is being reused

Currently when we decide to reuse xattr block we detect the case when
the last reference to xattr block is being dropped at the same time and
cancel the reuse attempt. Convert ext2 to a new scheme when as soon as
matching mbcache entry is found, we wait with dropping the last xattr
block reference until mbcache entry reference is dropped (meaning either
the xattr block reference is increased or we decided not to reuse the
block).

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-8-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>