inet: don't use sk_v6_rcv_saddr directly

When comparing two sockets we need to use inet6_rcv_saddr so we get a NULL
sk_v6_rcv_saddr if the socket isn't AF_INET6, otherwise our comparison function
can be wrong.

Fixes: 637bc8bb ("inet: reset tb->fastreuseport when adding a reuseport sk")
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>