os/LFNIndex: remove alt xattr after unlink

After we unlink, if the nlink on the inode is still non-zero, remove the
alt xattr.  We can *only* do this after the rename or unlink operation
because we don't want to leave a file system link in place without the
matching xattr; hence the fsync_dir() call.

Note that this might leak an alt xattr if we happen to fail after the
rename/unlink but before the removexattr is committed.  We'll fix that
next.

Signed-off-by: Sage Weil <sage@redhat.com>
os/LFNIndex: only consider alt xattr if nlink > 1

If we are doing a lookup, the main xattr fails, we'll check if there is an
alt xattr.  If it exists, but the nlink on the inode is only 1, we will
kill the xattr.  This cleans up the mess left over by an incomplete
lfn_unlink operation.

This resolves the problem with an lfn_link to a second long name that
hashes to the same short_name: we will ignore the old name the moment the
old link goes away.

Fixes: #8701
Signed-off-by: Sage Weil <sage@redhat.com>