Sanity check ICMP6 router advertisement packets

There is a chance a packet can slip by before we install the filter
on our socket listening for RAs, so add some basic sanity checking
to make sure we've recieved an RA.

Change-Id: I14cf84a0814896a41e00f50af376dfc4988d36cb
Fixes: 29586253
(cherry picked from commit a3621386)