[XFS] merge xfs_mount into xfs_fs_fill_super

xfs_mount is already pretty linux-specific so merge it into
xfs_fs_fill_super to allow for a more structured mount code in the next
patches. xfs_start_flags and xfs_finish_flags also move to xfs_super.c.

SGI-PV: 981951
SGI-Modid: xfs-linux-melb:xfs-kern:31189a

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