[XFS] Put the correct offset in dirent d_off

The recent filldir regression fix was not putting the correct d_off in
each dirent. This was resulting in incorrect cookies being passed to dmapi
ioctls and the wrong offset appearing in the dirents. readdir was
unaffected as the filp->f_pos was being updated with the correct offset
and this was being written into the last dirent in each buffer. Fix the
XFS code to do the right thing.

SGI-PV: 973746
SGI-Modid: xfs-linux-melb:xfs-kern:30240a

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