[XFS] add xfs_setup_devices helper

Split setting the block and sector size out of xfs_fs_fill_super into a
small helper to make xfs_fs_fill_super more readable.

SGI-PV: 981951
SGI-Modid: xfs-linux-melb:xfs-kern:31194a

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