zebra: advertise stale neighs if EVPN-MH is not enabled

zebra: advertise stale neighs if EVPN-MH is not enabled

With EVPN-MH, Type-2 routes are also used for MAC-IP syncing between
ES peers so a change was done to only treat REACHABLE local neigh
entries as local-active and advertise them as Type-2 routes i.e. STALE
neigh entries are no longer advertised as Type-2s.

This however exposed some unexpected problems with MLAG where a
secondary reboot followed by a primary reboot left a lot of neighs
in STALE state (on the primary) resulting in them not being
advertised. And remote routed traffic to those hosts being
blackholed in a sym-IRB setup.

This commit is a workaround to fix the regression (it doesn't fix
the underlying problems with entries not becoming REACHABLE; which
maybe a day-1 problem). The workaround is to continue advertising
STALE neighbors if EVPN-MH is not enabled.

Ticket: CM-30303

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