syscall: skip routing messages with mismatched version
syscall: skip routing messages with mismatched version

Skip routing messages with a mismatched version, rather than failing
and returning EINVAL. Only return EINVAL if we were unable to parse
any of the routing messages (presumably due to a version mismatch).

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/30340043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang