L2FWD:fix seq_num overwritten + validate l2fib entries when forwarding
l2_classify memeber table_index was overlaid over l2.l2fib_seq_num
which over written when table_index gets initialized in l2_input_classify
solved by overlaying both table_index and opaque_index as only one is used
seperated l2fib seq num from l2_input configs
for better handling of theoretical ABA issue where an entry for a deleted
interface is considered valid by the ager because a different interface with
same sw_if_index and seq_num was created before the ager got a chance to delete
Change-Id: I7b0eeded971627406f1c80834d7e02c0ebe62136
Signed-off-by: Eyal Bari <ebari@cisco.com>