Assign the higher metric value for Ipv6 default route learnt via RA m…

…essage (sonic-net#16367)

* Fix the Loopback0 IPv6 address of LC's in chassis not reachable from peer device's
* Assign the metric vaule for Ipv6 default route learnt via RA message to higher value so that BGP learnt default route is higher priority.

Signed-off-by: Abhishek Dosi <abdosi@microsoft.com>

files/dhcp/90-dhcp6-systcl.conf.j2

@@ -5,3 +5,4 @@ net.ipv6.conf.eth0.accept_ra = 0
 net.ipv6.conf.eth0.accept_ra_defrtr = 1
 net.ipv6.conf.eth0.accept_ra = 1
 {% endif %}
+net.ipv6.conf.eth0.ra_defrtr_metric=1996489704