zebra: fix EVPN svd based remote nh neigh del

zebra: fix EVPN svd based remote nh neigh del

In the context of SVD (Single VxLAN Device) for L3VNI,
the remote VTEP's nexthop is programmed neighbor entry against
SVD along with neighbor entry against SVI.

However, when L3VNI is removed or the VRF is disabled, all SVI
based remote nexthop neighbors are uninstalled and deleted.
The SVD based neigh entries remains in Zebra and the Kernel.
Subsequently, when reconfiguring L3VNI and relearning the same nexthop,
the neighbor entry is not programmed is because it is not removed
from Zebra SVD neighbor hash table, leading to the failure to
reprogram the entry.

With this fix, the SVD nexthop neigh entry is uninstalled
and deleted from Zebra and Kernel.

Ticket: #3729045

Testing:

borderleaf:# ip neigh show 2.2.2.2
2.2.2.2 dev vlan2560_l3 lladdr 00:01:00:00:1d:09 extern_learn NOARP proto zebra
2.2.2.2 dev vxlan99 lladdr 00:01:00:00:1d:09 extern_learn NOARP proto zebra

With the fix:

Zebra log shows both enties SVD (vxlan99) and SVI (vlan2560_l3)
neighbor entries are deleted.

2024/05/03 18:41:33.527125 ZEBRA: [NH6N7-54CD1] Tx RTM_DELNEIGH family
ipv4 IF vxlan99(16) Neigh 2.2.2.2 MAC  null flags 0x10 state 0x0
ext_flags 0x0
2024/05/03 18:41:33.527128 ZEBRA: [NH6N7-54CD1] Tx RTM_DELNEIGH family
ipv4 IF vlan2560_l3(18) Neigh 2.2.2.2 MAC  null flags 0x10 state 0x0
ext_flags 0x0

borderleaf:# ip neigh show 2.2.2.2
borderleaf:#

Signed-off-by: Chirag Shah <chirag@nvidia.com>
(cherry picked from commit fde6dd7bb9161cf58fdd8ca066cce77bad159c9d)