net: Make setsockopt() optlen be unsigned.

This provides safety against negative optlen at the type
level instead of depending upon (sometimes non-trivial)
checks against this sprinkled all over the the place, in
each and every implementation.

Based upon work done by Arjan van de Ven and feedback
from Linus Torvalds.

Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppol2tp.c

@@ -2179,7 +2179,7 @@ static int pppol2tp_session_setsockopt(struct sock *sk,
  * session or the special tunnel type.
  */
 static int pppol2tp_setsockopt(struct socket *sock, int level, int optname,
-			       char __user *optval, int optlen)
+			       char __user *optval, unsigned int optlen)
 {
 	struct sock *sk = sock->sk;
 	struct pppol2tp_session *session = sk->sk_user_data;