IB/umad: make sure write()s have sufficient data

Make sure that userspace passes in enough data when sending a MAD.  We
always copy at least sizeof (struct ib_user_mad) + IB_MGMT_RMPP_HDR
bytes from userspace, so anything less is definitely invalid.  Also,
if the length is less than this limit, it's possible for the second
copy_from_user() to get a negative length and trigger a BUG().
Signed-off-by: NRoland Dreier <rolandd@cisco.com>

drivers/infiniband/core/user_mad.c

@@ -312,7 +312,7 @@ static ssize_t ib_umad_write(struct file *filp, const char __user *buf,
 	int ret, length, hdr_len, copy_offset;
 	int rmpp_active = 0;
 
-	if (count < sizeof (struct ib_user_mad))
+	if (count < sizeof (struct ib_user_mad) + IB_MGMT_RMPP_HDR)
 		return -EINVAL;
 
 	length = count - sizeof (struct ib_user_mad);