zebra: reinstall missing peer-sync flag

zebra: reinstall missing peer-sync flag

If a netlink/dp notification is rxed for a neigh without the peer-sync
flag FRR re-installs the entry with the right flags. This change is
needed to handle cases where the dataplane and FRR may fall out of
sync because of neigh learning on the network ports (i.e. via
the VxLAN).

Ticket: CM-30693
The problem was found during VM mobility "torture" tests where 100s
of extended VM moves were done.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>