xfrm: Fix ESN sequence number handling for IPsec GSO packets.

When IPsec offloading was introduced, we accidentally incremented
the sequence number counter on the xfrm_state by one packet
too much in the ESN case. This leads to a sequence number gap of
one packet after each GSO packet. Fix this by setting the sequence
number to the correct value.

Fixes: d7dbefc4 ("xfrm: Add xfrm_replay_overflow functions for offloading")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>