datapath-windows: Remove IP neighbor entries when internal adapter is down.

datapath-windows: Remove IP neighbor entries when internal adapter is down.

Remove the IP neighboring entries when adapter is down,
so that when 'OVS_IPHELPER_INSTANCE' is deleted, no stale entries
are present

Also fix accessing iphelper instance without acquiring the lock.

Signed-off-by: Anand Kumar <kumaranand@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Tested-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>