[UDP]: saddr_cmp function should take const socket pointers

This also kills a warning while building ipv6:

net/ipv6/udp.c: In function ‘udp_v6_get_port’:
net/ipv6/udp.c:66: warning: passing argument 3 of ‘udp_get_port’ from incompatible pointer type

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