[SCSI] sg: Free data buffers after calling blk_rq_unmap_user

Running sg_luns on s390x with CONFIG_DEBUG_PAGEALLOC enabled fails
with EFAULT from the SG_IO ioctl. The EFAULT is the result from
copy_to_user failing in this call chain:

sg_ioctl
sg_new_read
sg_finish_rem_req
blk_rq_unmap_user
__blk_rq_unmap_user
bio_uncopy_user
__bio_copy_iov
copy_to_user

The sg driver calls sg_remove_scat to free the memory pages before
calling blk_rq_unmap_user that tries to copy the data back to
userspace. Change the order to first call blk_rq_unmap_user before
freeing the pages in sg_remove_scat.
Acked-by: NFUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: stable@kernel.org
Signed-off-by: NChristof Schmitt <christof.schmitt@de.ibm.com>
Acked-by: NDouglas Gilbert <dgilbert@interlog.com>
Signed-off-by: NJames Bottomley <James.Bottomley@suse.de>

drivers/scsi/sg.c

@@ -1708,11 +1708,6 @@ static int sg_finish_rem_req(Sg_request * srp)
 	Sg_scatter_hold *req_schp = &srp->data;
 
 	SCSI_LOG_TIMEOUT(4, printk("sg_finish_rem_req: res_used=%d\n", (int) srp->res_used));
-	if (srp->res_used)
-		sg_unlink_reserve(sfp, srp);
-	else
-		sg_remove_scat(req_schp);
-
 	if (srp->rq) {
 		if (srp->bio)
 			ret = blk_rq_unmap_user(srp->bio);
@@ -1720,6 +1715,11 @@ static int sg_finish_rem_req(Sg_request * srp)
 		blk_put_request(srp->rq);
 	}
 
+	if (srp->res_used)
+		sg_unlink_reserve(sfp, srp);
+	else
+		sg_remove_scat(req_schp);
+
 	sg_remove_request(sfp, srp);
 
 	return ret;