[SCSI] sg: fix bio leak with a detached device

After blk_rq_map_user is successful, if we find that a device is
unavailable (was detached), we must call blk_end_request_all to free
bio(s) before blk_rq_unmap_user and blk_put_request.

Reported-by: "Dailey, Nate" <Nate.Dailey@stratus.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Tested-by: "Dailey, Nate" <Nate.Dailey@stratus.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>