bgpd: Mac rescan on interface up/down efficency improvements

bgpd: Mac rescan on interface up/down efficency improvements

On interface up/down, bgp stores the mac address of the interface
in a bgp_mac_hash table entry and then initiates a rescan
of the evpn l2vpn table.  The problem with this scan is that
it is looking at every item in the table when only 1 mac
has changed.  So every up/down event causes some major trauma
in the bgp_update processing.

Modify the mac scanning such that we know the mac that is changed
and as such we should reprocess those entries only.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>