[XFS] Move platform specific mount option parse out of core XFS code

Mount option parsing is platform specific. Move it out of core code into
the platform specific superblock operation file.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30012a

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