When the IP layer calls back into the SCTP layer to perform the SCTP

When the IP layer calls back into the SCTP layer to perform the SCTP
checksum computation, do not assume that the IP header chain and the
SCTP common header are in contiguous memory although the SCTP lays
out the mbuf chains that way. If there are IP-level options inserted
by the IP layer, the constraint is not fulfilled anymore.

This issues was found by running syzkaller. Thanks to markj@ who is
running an instance which also provides kernel dumps. This allowed me
to find this issue.

MFC after:		3 days