net: gro: change skb_gro_network_header()

Change skb_gro_network_header() to accept a const sk_buff
and to no longer check if frag0 is NULL or not.

This allows to remove skb_gro_frag0_invalidate()
which is seen in profiles when header-split is enabled.

sk_buff parameter is constified for skb_gro_header_fast(),
inet_gro_compute_pseudo() and ip6_gro_compute_pseudo().

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