os/LFNIndex: only consider alt xattr if nlink > 1

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>