sctp: bring inet(6)_skb_parm back to sctp_input_cb

inet(6)_skb_parm was removed from sctp_input_cb by Commit a1dd2cf2f1ae
("sctp: allow changing transport encap_port by peer packets"), as it
thought sctp_input_cb->header is not used any more in SCTP.

syzbot reported a crash:

  [ ] BUG: KASAN: use-after-free in decode_session6+0xe7c/0x1580
  [ ]
  [ ] Call Trace:
  [ ]  <IRQ>
  [ ]  dump_stack+0x107/0x163
  [ ]  kasan_report.cold+0x1f/0x37
  [ ]  decode_session6+0xe7c/0x1580
  [ ]  __xfrm_policy_check+0x2fa/0x2850
  [ ]  sctp_rcv+0x12b0/0x2e30
  [ ]  sctp6_rcv+0x22/0x40
  [ ]  ip6_protocol_deliver_rcu+0x2e8/0x1680
  [ ]  ip6_input_finish+0x7f/0x160
  [ ]  ip6_input+0x9c/0xd0
  [ ]  ipv6_rcv+0x28e/0x3c0

It was caused by sctp_input_cb->header/IP6CB(skb) still used in sctp rx
path decode_session6() but some members overwritten by sctp6_rcv().

This patch is to fix it by bring inet(6)_skb_parm back to sctp_input_cb
and not overwriting it in sctp4/6_rcv() and sctp_udp_rcv().

Reported-by:  <syzbot+5be8aebb1b7dfa90ef31@syzkaller.appspotmail.com>
Fixes: a1dd2cf2f1ae ("sctp: allow changing transport encap_port by peer packets")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Link: https://lore.kernel.org/r/136c1a7a419341487c504be6d1996928d9d16e02.1604472932.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>