scsi: merge scsi_init_sgtable into scsi_init_io

scsi_init_io is the only caller of scsi_init_sgtable.  Merge the two
function to make upcoming changes a little easier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>