net: bridge: fix flags interpretation for extern learn fdb entries

Ignore fdb flags when adding port extern learn entries and always set
BR_FDB_LOCAL flag when adding bridge extern learn entries. This is
closest to the behaviour we had before and avoids breaking any use cases
which were allowed.

This patch fixes iproute2 calls which assume NUD_PERMANENT and were
allowed before, example:
$ bridge fdb add 00:11:22:33:44:55 dev swp1 extern_learn

Extern learn entries are allowed to roam, but do not expire, so static
or dynamic flags make no sense for them.

Also add a comment for future reference.

Fixes: eb100e0e ("net: bridge: allow to add externally learned entries from user-space")
Fixes: 0541a629

 ("net: bridge: validate the NUD_PERMANENT bit when adding an extern_learn FDB entry")
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20210810110010.43859-1-razor@blackwall.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>