xfrm: introduce xfrm_trans_queue_net

This will be used by TCP encapsulation to write packets to the encap
socket without holding the user socket's lock. Without this reinjection,
we're already holding the lock of the user socket, and then try to lock
the encap socket as well when we enqueue the encrypted packet.

While at it, add a BUILD_BUG_ON like we usually do for skb->cb, since
it's missing for struct xfrm_trans_cb.

Co-developed-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>