rds: rds_msg_zcopy should return error of null rm->data.op_mmp_znotifier

if either or both of MSG_ZEROCOPY and SOCK_ZEROCOPY have not been
specified, the rm->data.op_mmp_znotifier allocation will be skipped.
In this case, it is invalid ot pass down a cmsghdr with
RDS_CMSG_ZCOPY_COOKIE, so return EINVAL from rds_msg_zcopy for this
case.

Reported-by: [e-mail]
Fixes: 0cebaccef3ac ("rds: zerocopy Tx support.")
Signed-off-by: Sowmini Varadhan <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Acked-by: Santosh Shilimkar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/rds/send.c

···
 {
 	u32 *cookie;
 
-	if (cmsg->cmsg_len < CMSG_LEN(sizeof(*cookie)))
+	if (cmsg->cmsg_len < CMSG_LEN(sizeof(*cookie)) ||
+	    !rm->data.op_mmp_znotifier)
 		return -EINVAL;
 	cookie = CMSG_DATA(cmsg);
 	rm->data.op_mmp_znotifier->z_cookie = *cookie;