esp6: Fix integrity verification when ESN are used

When handling inbound packets, the two halves of the sequence number
stored on the skb are already in network order.

Fixes: 000ae7b2 ("esp6: Switch to new AEAD interface")
Signed-off-by: Tobias Brunner <tobias@strongswan.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>