Merge branch 'inet-prevent-skb-changes-in-udp-4-6-_lib_lookup_skb'

Eric Dumazet says:

====================
inet: prevent skb changes in udp{4|6}_lib_lookup_skb()

This came while reviewing Alexander Lobakin patch against UDP GRO:

We want to make sure skb wont be changed by these helpers
while it is owned by GRO stack.
====================

Link: https://lore.kernel.org/r/20201109231349.20946-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>