ospf6d: fix ifmtu settings when kernel changes values

ospf6d: fix ifmtu settings when kernel changes values

Problem reported by customer that if an mtu value was set in the kernel,
quagga/frr would get very confused about what had been configured and
what had been learned.  This caused peers to not be successfully established.
Resolved by keeping a configuration value separate than the operational value
and set the operational accordingly.  If configured, it wins unless the config
defines a value that is higher than the kernel supports.

Ticket: CM-16876
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed By: CCR-6399
Testing Done: Manual testing successful, submitter tested, ospf-smoke completed
with no new failures.