fib: fix the drop counter for ipv6 RPF failures

fib: fix the drop counter for ipv6 RPF failures

Type: fix

the only change to the mfib forwarding node is to set the error code, the rest is checkstyle formatting.

The traces previously showed some bogus reason:

00:04:27:325550: ip6-mfib-forward-rpf
  entry 10 itf -1 flags
00:04:27:325551: ip6-drop
    fib:0 adj:10 flow:0
  UDP: fe80::b203:eaff:fe02:604 -> ff02::1:2
    tos 0x00, flow label 0x651ed, hop limit 1, payload length 64
  UDP: 546 -> 547
    length 64, checksum 0xec9a
00:04:27:325551: error-drop
  rx:GigabitEthernet6/0/0
00:04:27:325553: drop
  ip6-input: drops due to concurrent reassemblies limit

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I294684c36edc346b4ebdd83ba66888b3b2197704