sctp: define sctp_packet_gso_append to build GSO frames

Now sctp GSO uses skb_gro_receive() to append the data into head
skb frag_list. However it actually only needs very few code from
skb_gro_receive(). Besides, NAPI_GRO_CB has to be set while most
of its members are not needed here.

This patch is to add sctp_packet_gso_append() to build GSO frames
instead of skb_gro_receive(), and it would avoid many unnecessary
checks and make the code clearer.

Note that sctp will use page frags instead of frag_list to build
GSO frames in another patch. But it may take time, as sctp's GSO
frames may have different size. skb_segment() can only split it
into the frags with the same size, which would break the border
of sctp chunks.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>