sg: convert the indirect IO path to use the block layer

sg: convert the indirect IO path to use the block layer

This patch converts the indirect IO path (including mmap IO and old
struct sg_header) to use the block layer functions (blk_get_request,
blk_execute_rq_nowait, blk_rq_map_user, etc) instead of
scsi_execute_async().

[Jens: fixed compile error with SCSI logging enabled]

Signed-off-by: FUJITA Tomonori <[email protected]>
Signed-off-by: Douglas Gilbert <[email protected]>
Cc: Mike Christie <[email protected]>
Cc: James Bottomley <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>