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>