ipv4: potential underflow in compat_ip_setsockopt()

The value of "n" is capped at 0x1ffffff but it checked for negative
values.  I don't think this causes a problem but I'm not certain and
it's harmless to prevent it.

Fixes: 2e041728 ("ipv4: do compat setsockopt for MCAST_MSFILTER directly")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -1347,8 +1347,8 @@ int compat_ip_setsockopt(struct sock *sk, int level, int optname,
 	{
 		const int size0 = offsetof(struct compat_group_filter, gf_slist);
 		struct compat_group_filter *gf32;
+		unsigned int n;
 		void *p;
-		int n;
 
 		if (optlen < size0)
 			return -EINVAL;