router: fix Lan host reachibility due to identical RIO and PIO prefixes (FS#3056)

router: fix Lan host reachibility due to identical RIO and PIO prefixes (FS#3056)

odhcpd includes RIO RA options according to requirement L3 in RFC7084.
However if the delegated prefix length received on the wan is equal to the
downstream delegated prefix length on the Lan this may pollute the
routing table of type C hosts as the RIO routing entry can take
precedence of the PIO routing entry meaning all traffic for the on link
hosts will go via the router iso direct on link communication.
If the traffic is dropped in the router hosts are unreachable; therefore
don't include RIO options with prefixes and prefix length identical to
those in a PIO RA option

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>