BACKPORT: net: add skb_set_end_offset() helper

We have multiple places where this helper is convenient,
and plan using it in the following patch.

Bug: 254441685
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 763087da)
[Lee: Solves a dependency for the next Fixes: patch]
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I044e7020f9d0b85073253fbd4629cf97bbd38736
Signed-off-by: Lee Jones <joneslee@google.com>