Fix handling of ancillary data on non-AF_UNIX Linux sockets.

After r340674, the "continue" would restart the loop without having
updated clen, resulting in an infinite loop. Restore the old behaviour
of simply ignoring all control messages on such sockets, since we
currently only implement handling for AF_UNIX-specific messages.

Reported by: syzkaller
Reviewed by: tijl
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26093