zebra: clear evpn dup-addr return error-msg when there is no vni (backport #16261)

[mergify[bot]]

clear evpn dup-addr cli returns error-msg for below conditions,

• If evpn is not enabled &
• If there is no VNI exists.

supported command:

clear evpn dup-addr vni <vni-id>

Testing:

bharat# clear evpn dup-addr vni 20
Error type: validation
Error description: % EVPN not enabled

bharat# clear evpn dup-addr vni 20
Error type: validation
Error description: % VNI 20 does not exist

Signed-off-by: Sindhu Parvathi Gopinathan's sgopinathan@nvidia.com Signed-off-by: Chirag Shah chirag@nvidia.com

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This is an automatic backport of pull request #16261 done by Mergify.