xfrm: Fix GSO for IPsec with GRE tunnel.

We reset the encapsulation field of the skb too early
in xfrm_output. As a result, the GRE GSO handler does
not segment the packets. This leads to a performance
drop down. We fix this by resetting the encapsulation
field right before we do the transformation, when
the inner headers become invalid.

Fixes: f1bd7d65 ("xfrm: Add encapsulation header offsets while SKB is not encrypted")
Reported-by: Vicente De Luca <vdeluca@zendesk.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>