block,scsi: fixup blk_get_request dead queue scenarios

The blk_get_request function may fail in low-memory conditions or during
device removal (even if __GFP_WAIT is set). To distinguish between these
errors, modify the blk_get_request call stack to return the appropriate
ERR_PTR. Verify that all callers check the return status and consider
IS_ERR instead of a simple NULL pointer check.

For consistency, make a similar change to the blk_mq_alloc_request leg
of blk_get_request.  It may fail if the queue is dead, or the caller was
unwilling to wait.

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Jiri Kosina <jkosina@suse.cz> [for pktdvd]
Acked-by: Boaz Harrosh <bharrosh@panasas.com> [for osd]
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/scsi/sg.c

@@ -1711,9 +1711,9 @@ sg_start_req(Sg_request *srp, unsigned char *cmd)
 	}
 
 	rq = blk_get_request(q, rw, GFP_ATOMIC);
-	if (!rq) {
+	if (IS_ERR(rq)) {
 		kfree(long_cmdp);
-		return -ENOMEM;
+		return PTR_ERR(rq);
 	}
 
 	blk_rq_set_block_pc(rq);