net: add skb_set_end_offset() helper

net: add skb_set_end_offset() helper

BugLink: https://bugs.launchpad.net/bugs/1969110
commit 763087dab97547230a6807c865a6a5ae53a59247 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>
(cherry picked from commit 51e458fc0ca63a0a8e8946dfad19535ae7bb2772)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>