bcachefs: fsck: Improve hash_check_key()

hash_check_key() checks and repairs the hash table btrees: dirents and
xattrs are open addressing hash tables.

We recently had a corruption reported where the hash type on an inode
somehow got flipped, which made the existing dirents invisible and
allowed new ones to be created with the same name.

Now, hash_check_key() can repair duplicates: it will delete one of them,
if it has an xattr or dangling dirent, but if it has two valid dirents
one of them gets renamed.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>