linux-cp: ignore neighbors if ip addr is multicast

linux-cp: ignore neighbors if ip addr is multicast

Type: improvement

When dump of neighbors is requested, the replies will also include
neighbor entries for IPv6 multicast addresses:

  GigabitEthernet0/8/0  S           ff02::16  33:33:00:00:00:16
  GigabitEthernet0/8/0  S  ff02::1:ff76:7135  33:33:ff:76:71:35
  GigabitEthernet0/8/0  S            ff02::2  33:33:00:00:00:02

Such entries are not reported in netlink notification messages and
VPP is unlikely to use these.

With this change, ignore neighbor entries when the IP address is a
multicast address.

Signed-off-by: Alexander Chernavin <[email protected]>
Change-Id: Ic712aa4904f1d559f31fd89ff4541268e2340f84