inet_sdif() does not modify the skb.

This will permit propagating the const qualifier in
udp{4|6}_lib_lookup_skb() functions.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NAlexander Lobakin <alobakin@pm.me>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>