bgpd: Inconsistencies in SNT counters with default-originate

bgpd: Inconsistencies in SNT counters with default-originate

Description:
Change is intended for fixing the inconsistencies present
while adjusting the SNT counters with default originate.

- SNT counter gets incremented on every change of policy associated
  with default-originate, leading to inconsistencies.
- This fix has been added to ensure that the SNT counters gets
  incremented and decremented only once during the creation and
  deletion workflow of default-originate, and prevents
  incrementing the counter during update flow.

Co-authored-by: Abhinay Ramesh <rabhinay@vmware.com>
Signed-off-by: Iqra Siddiqui <imujeebsiddi@vmware.com>