bridge: mcast: Allow user space to specify MDB entry routing protocol

Add the 'MDBE_ATTR_RTPORT' attribute to allow user space to specify the
routing protocol of the MDB port group entry. Enforce a minimum value of
'RTPROT_STATIC' to prevent user space from using protocol values that
should only be set by the kernel (e.g., 'RTPROT_KERNEL'). Maintain
backward compatibility by defaulting to 'RTPROT_STATIC'.

The protocol is already visible to user space in RTM_NEWMDB responses
and notifications via the 'MDBA_MDB_EATTR_RTPROT' attribute.

The routing protocol allows a routing daemon to distinguish between
entries configured by it and those configured by the administrator. Once
MDB flush is supported, the protocol can be used as a criterion
according to which the flush is performed.

Examples:

 # bridge mdb add dev br0 port dummy10 grp 239.1.1.1 permanent proto kernel
 Error: integer out of range.

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

 # bridge mdb add dev br0 port dummy10 grp 239.1.1.1 src 192.0.2.1 permanent proto zebra

 # bridge mdb add dev br0 port dummy10 grp 239.1.1.2 permanent source_list 198.51.100.1,198.51.100.2 filter_mode include proto 250

 # bridge -d mdb show
 dev br0 port dummy10 grp 239.1.1.2 src 198.51.100.2 permanent filter_mode include proto 250
 dev br0 port dummy10 grp 239.1.1.2 src 198.51.100.1 permanent filter_mode include proto 250
 dev br0 port dummy10 grp 239.1.1.2 permanent filter_mode include source_list 198.51.100.2/0.00,198.51.100.1/0.00 proto 250
 dev br0 port dummy10 grp 239.1.1.1 src 192.0.2.1 permanent filter_mode include proto zebra
 dev br0 port dummy10 grp 239.1.1.1 permanent filter_mode exclude proto static

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>