MFC r339024:

Fix the handling of ancillary data for SCTP socket. Implement
sctp_process_cmsgs_for_init() and sctp_findassociation_cmsgs()
similar to sctp_find_cmsg() to improve consistency and avoid
the signed/unsigned issues in sctp_process_cmsgs_for_init()
and sctp_findassociation_cmsgs().

Thanks to andrew@ for reporting the problem he found using
syzcaller.