sctp: fix missing encap_port propagation for GSO fragments

sctp: fix missing encap_port propagation for GSO fragments

encap_port in SCTP_INPUT_CB(skb) is used by sctp_vtag_verify() for
SCTP-over-UDP processing. In the GSO case, it is only set on the head
skb, while fragment skbs leave it 0.

This results in fragment skbs seeing encap_port == 0, breaking
SCTP-over-UDP connections.

Fix it by propagating encap_port from the head skb cb when initializing
fragment skbs in sctp_inq_pop().

Fixes: 046c052b475e ("sctp: enable udp tunneling socks")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://patch.msgid.link/ea65ed61b3598d8b4940f0170b9aa1762307e6c3.1776017631.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>