[XFS] kill BMAPI_UNWRITTEN

There is no reason to go through xfs_iomap for the BMAPI_UNWRITTEN because
it has nothing in common with the other cases. Instead check for the
shutdown filesystem in xfs_end_bio_unwritten and perform a direct call to
xfs_iomap_write_unwritten (which should be renamed to something more
sensible one day)

SGI-PV: 970241
SGI-Modid: xfs-linux-melb:xfs-kern:29681a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>