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: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>