ethernet: fix DMAC check and skip unnecessary ones (VPP-1868)

ethernet: fix DMAC check and skip unnecessary ones (VPP-1868)

Fix and optimize DMAC check in ethernet-input node to utilize NIC or
driver which support L3 DMAC-filtering mode so that DMAC check can be
bypassed safely for interfaces/sub-interfaces in L3 mode.
Checking of interface in L3-DMAC-filtering state to avoid DMAC check
require the following:
a) Fix interface driver init sequence for devices which supports L3
   DMAC-filtering to indicate its capability and initialize interface
   to L3 DMAC-filtering state.
b) Fix ethernet_set_flags() function and its associated callback
   flags_change() functions registered by various drivers in interface
   infra to provide proper L3 DMAC filtering status.
Maintain interface/sub-interface L3 config count so DMAC checks can be
bypassed if L3 forwarding is not setup on any main/sub-interfaces.

Type: fix
Ticket: VPP-1868

Signed-off-by: John Lo <loj@cisco.com>
Change-Id: I204d90459c13e9e486cfcba4e64e3d479bc9f2ae