afs: Fix comment in afs_do_lookup()

Fix the comment in afs_do_lookup() that says that slot 0 is used for the
fid being looked up and slot 1 is used for the directory.  It's actually
done the other way round.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org