net: add skb_set_end_offset() helper

commit 763087da upstream.

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>