[XFS] use inode_change_ok for setattr permission checking

Instead of implementing our own checks use inode_change_ok to check for
necessary permission in setattr.  There is a slight change in behaviour
as inode_change_ok doesn't allow i_mode updates to add the suid or sgid
without superuser privilegues while the old XFS code just stripped away
those bits from the file mode.

(First sent on Semptember 29th)

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