[XFS] Show all mount args in /proc/mounts

There are several mount options that don't show up in /proc/mounts. Add
them in and clean up the showargs code at the same time.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30004a

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