inet: udp{4|6}_lib_lookup_skb() skb argument is const

The skb is needed only to fetch the keys for the lookup.

Both functions are used from GRO stack, we do not want
accidental modification of the skb.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>