bsg: add sg_io_v4 structure

This patch adds sg_io_v4 structure that Doug proposed last month.

There's one major change from the RFC. I dropped iovec, which needs
compat stuff. The bsg code simply calls blk_rq_map_user against
dout_xferp/din_xferp. So if possible, the page frames are directly
mapped. If not possible, the block layer allocates new page frames and
does memory copies.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>