virtio_blk: Fix an SG_IO regression

virtio_blk: Fix an SG_IO regression

Avoid that submitting an SG_IO ioctl triggers a kernel oops that
is preceded by:

usercopy: kernel memory overwrite attempt detected to (null) (<null>) (6 bytes)
kernel BUG at mm/usercopy.c:72!

Reported-by: Dann Frazier <[email protected]>
Fixes: commit ca18d6f769d2 ("block: Make most scsi_req_init() calls implicit")
Signed-off-by: Bart Van Assche <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Dann Frazier <[email protected]>
Cc: <[email protected]> # v4.13
Reviewed-by: Christoph Hellwig <[email protected]>
Moved virtblk_initialize_rq() inside CONFIG_VIRTIO_BLK_SCSI.

Signed-off-by: Jens Axboe <[email protected]>