Revert "ip6_tunnel: make sure to pull inner header in __ip6_tnl_rcv()"

This reverts commit af6b5c50 which is
commit 8d975c15 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I4da6051a5797a4b9a51b899b7eaf6d1d4d327bed
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>