ip_gre: validate csum_start only on pull

The GRE tunnel device can pull existing outer headers in ipge_xmit.
This is a rare path, apparently unique to this device. The below
commit ensured that pulling does not move skb->data beyond csum_start.

But it has a false positive if ip_summed is not CHECKSUM_PARTIAL and
thus csum_start is irrelevant.

Refine to exclude this. At the same time simplify and strengthen the
test.

Simplify, by moving the check next to the offending pull, making it
more self documenting and removing an unnecessary branch from other
code paths.

Strengthen, by also ensuring that the transport header is correct and
therefore the inner headers will be after skb_reset_inner_headers.
The transport header is set to csum_start in skb_partial_csum_set.

Link: https://lore.kernel.org/netdev/YS+h%2FtqCJJiQei+W@shredder/
Fixes: 1d011c48

 ("ip_gre: add validation for csum_start")
Reported-by: Ido Schimmel <idosch@idosch.org>
Suggested-by: Alexander Duyck <alexander.duyck@gmail.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>