esp: Fix ESN generation under UDP encapsulation

Blair Steven noticed that ESN in conjunction with UDP encapsulation
is broken because we set the temporary ESP header to the wrong spot.

This patch fixes this by first of all using the right spot, i.e.,
4 bytes off the real ESP header, and then saving this information
so that after encryption we can restore it properly.

Fixes: 7021b2e1

 ("esp4: Switch to new AEAD interface")
Reported-by: Blair Steven <Blair.Steven@alliedtelesis.co.nz>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>