udp: use sk_filter_trim_cap for udp{,6}_queue_rcv_skb

udp: use sk_filter_trim_cap for udp{,6}_queue_rcv_skb

After b9ce4a78686d ("udp: prevent bugcheck if filter truncates packet
too much"), there followed various other fixes for similar cases such
as c98e58bf49dd ("rose: limit sk_filter trim to payload").

Latter introduced a new helper sk_filter_trim_cap(), where we can pass
the trim limit directly to the socket filter handling. Make use of it
here as well with sizeof(struct udphdr) as lower cap limit and drop the
extra skb->len test in UDP's input path.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>