[XFS] keep i_nlink updated and use proper accessors

To get the read-only bind mounts in -mm to work correctly with XFS we need
to call the drop_nlink and inc_nlink helpers to monitor the link count.
Add calls to these to xfs_bumplink and xfs_droplink and stop copying over
di_nlink to i_nlink in xfs_validate_fields and vn_revalidate.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30392a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>