xfrm: Add support for network devices capable of removing the ESP trailer

xfrm: Add support for network devices capable of removing the ESP trailer

In conjunction with crypto offload [1], removing the ESP trailer by
hardware can potentially improve the performance by avoiding (1) a
cache miss incurred by reading the nexthdr field and (2) the necessity
to calculate the csum value of the trailer in order to keep skb->csum
valid.

This patch introduces the changes to the xfrm stack and merely serves
as an infrastructure. Subsequent patch to mlx5 driver will put this to
a good use.

[1] https://www.mail-archive.com/[email protected]/msg175733.html

Signed-off-by: Yossi Kuperman <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>