bridge: mdb: Add routing protocol support

bridge: mdb: Add routing protocol support

Allow user space to specify the routing protocol of the MDB port group
entry by adding the 'MDBE_ATTR_RTPROT' attribute to the
'MDBA_SET_ENTRY_ATTRS' nest.

Examples:

 # bridge mdb add dev br0 port dummy10 grp 239.1.1.1 permanent proto zebra

 # bridge mdb add dev br0 port dummy10 grp 239.1.1.2 permanent

 # bridge -d mdb show
 dev br0 port dummy10 grp 239.1.1.2 permanent filter_mode exclude proto static
 dev br0 port dummy10 grp 239.1.1.1 permanent filter_mode exclude proto zebra

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David Ahern <dsahern@kernel.org>