syscall: fix ParseRoutingSockaddr with unexpected submessages

No easy way to test (would have to actually trigger some routing
events from kernel) but the code is clearly wrong as written.
If the header says there is a submessage, we need to at least
skip over its bytes, not just continue to the next iteration.

Fixes issue 8203.

LGTM=r
R=r
CC=golang-codereviews, mikioh.mikioh, p
https://codereview.appspot.com/164140044