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>