xfrm: fix GRO for !CONFIG_NETFILTER

In xfrm_input() when called from GRO, async == 0, and we end up
skipping the processing in xfrm4_transport_finish(). GRO path will
always skip the NF_HOOK, so we don't need the special-case for
!NETFILTER during GRO processing.

Fixes: 7785bba2 ("esp: Add a software GRO codepath")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>