nvme: Fix nvme_get/set_features() with a NULL result pointer

BugLink: https://bugs.launchpad.net/bugs/1664602

nvme_set_features() callers seem to expect that passing NULL as the
result pointer is acceptable.  Teach nvme_set_features() not to try to
write to the NULL address.

For symmetry, make the same change to nvme_get_features(), despite the
fact that all current callers pass a valid result pointer.

I assume that this bug hasn't been reported in practice because
the callers that pass NULL are all in the SCSI translation layer
and no one uses the relevant operations.

Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit 9b47f77a)
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-By: AceLan Kao <acelan.kao@canonical.com>
Acked-By: Shrirang Bagul <shrirang.bagul@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/nvme/host/core.c

@@ -320,7 +320,7 @@ int nvme_get_features(struct nvme_ctrl *dev, unsigned fid, unsigned nsid,
 
 	ret = __nvme_submit_sync_cmd(dev->admin_q, &c, &cqe, NULL, 0, 0,
 			NVME_QID_ANY, 0, 0);
-	if (ret >= 0)
+	if (ret >= 0 && result)
 		*result = le32_to_cpu(cqe.result);
 	return ret;
 }
@@ -340,7 +340,7 @@ int nvme_set_features(struct nvme_ctrl *dev, unsigned fid, unsigned dword11,
 
 	ret = __nvme_submit_sync_cmd(dev->admin_q, &c, &cqe, NULL, 0, 0,
 			NVME_QID_ANY, 0, 0);
-	if (ret >= 0)
+	if (ret >= 0 && result)
 		*result = le32_to_cpu(cqe.result);
 	return ret;
 }