[XFS] Trivial xfs_remove comment fixup

The dp to ip comment should be for the unconditional xfs_droplink call,
and the "." link obviously only exists for directories, so it should be in
the is_dir conditional.

SGI-PV: 987246

SGI-Modid: xfs-linux-melb:xfs-kern:32374a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>