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>

net/rxrpc/af_rxrpc.c

@@ -507,7 +507,7 @@ out:
  * set RxRPC socket options
  */
 static int rxrpc_setsockopt(struct socket *sock, int level, int optname,
-			    char __user *optval, int optlen)
+			    char __user *optval, unsigned int optlen)
 {
 	struct rxrpc_sock *rx = rxrpc_sk(sock->sk);
 	unsigned min_sec_level;