inet: constify inet_sdif() argument

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