sg_write_xfer was used to copy data from user space for WRITE
commands. blk_rq_map_user_iov and blk_rq_map_user do the job so
sg_write_xfer does nothing useful.
Signed-off-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>