[XFS] kill struct xfs_mount_args

No need to parse the mount option into a structure before applying it to
struct xfs_mount.

The content of xfs_start_flags gets merged into xfs_parseargs. Calls
inbetween don't care and can use mount members instead of the args struct.

This patch uncovered that the mount option for shared filesystems wasn't
ever exposed on Linux. The code to handle it is #if 0'ed in this patch
pending a decision on this feature. I'll send a writeup about it to the
list soon.

SGI-PV: 987246

SGI-Modid: xfs-linux-melb:xfs-kern:32371a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>