tcp: gro: micro optimizations in tcp[4]_gro_complete()

In tcp_gro_complete() :

Moving the skb->inner_transport_header setting
allows the compiler to reuse the previously loaded value
of skb->transport_header.

Caching skb_shinfo() avoids duplications as well.

In tcp4_gro_complete(), doing a single change on
skb_shinfo(skb)->gso_type also generates better code.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>