xfs_mntupdate already is completely Linux specific due to the VFS flags
passed in, so it might aswell be merged into xfs_fs_remount.

SGI-PV: 981498
SGI-Modid: xfs-linux-melb:xfs-kern:31185a
Signed-off-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>