[XFS] Fix regression introduced by remount fixup

Logically we would return an error in xfs_fs_remount code to prevent users
from believing they might have changed mount options using remount which
can't be changed.

But unfortunately mount(8) adds all options from mtab and fstab to the
mount arguments in some cases so we can't blindly reject options, but have
to check for each specified option if it actually differs from the
currently set option and only reject it if that's the case.

Until that is implemented we return success for every remount request, and
silently ignore all options that we can't actually change.

SGI-PV: 985710

SGI-Modid: xfs-linux-melb:xfs-kern:31908a

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