bridge: add support for dynamic fdb entries

bridge: add support for dynamic fdb entries

This patch is a follow up to the recently added
'static' fdb option.

It introduces a new option 'dynamic' which adds
dynamic fdb entries with NUD_REACHABLE.

$bridge fdb add 00:01:02:03:04:06 dev eth0 master dynamic

$bridge fdb show
00:01:02:03:04:06 dev eth0

This patch also documents all fdb types. Removes 'temp'
from usage message since it is now replaced by 'static'.
'temp' still works and is synonymous with static.

Signed-off-by: Wilson Kok <wkok@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>