sctp: add encap_err_lookup for udp encap socks

As it says in rfc6951#section-5.5:

  "When receiving ICMP or ICMPv6 response packets, there might not be
   enough bytes in the payload to identify the SCTP association that the
   SCTP packet triggering the ICMP or ICMPv6 packet belongs to.  If a
   received ICMP or ICMPv6 packet cannot be related to a specific SCTP
   association or the verification tag cannot be verified, it MUST be
   discarded silently.  In particular, this means that the SCTP stack
   MUST NOT rely on receiving ICMP or ICMPv6 messages.  Implementation
   constraints could prevent processing received ICMP or ICMPv6
   messages."

ICMP or ICMPv6 packets need to be handled, and this is implemented by
udp encap sock .encap_err_lookup function.

The .encap_err_lookup function is called in __udp(6)_lib_err_encap()
to confirm this path does need to be updated. For sctp, what we can
do here is check if the corresponding asoc and transport exist.

Note that icmp packet process for sctp over udp is done by udp sock
.encap_err_lookup(), and it means for now we can't do as much as
sctp_v4/6_err() does. Also we can't do the two mappings mentioned
in rfc6951#section-5.5.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>