net/mlx5: Bridge, snoop igmp/mld packets

Handle SWITCHDEV_ATTR_ID_BRIDGE_MC_DISABLED attribute notification to
dynamically toggle bridge multicast offload. Set new
MLX5_ESW_BRIDGE_MCAST_FLAG bridge flag when multicast offload is enabled.
Put multicast-specific code into new bridge_mcast.c file.

When initializing bridge multicast pipeline create a static rule for
snooping on IGMP traffic and three rules for snooping on MLD traffic (for
query, report and done message types). Note that matching MLD traffic
requires having flexparser MLX5_FLEX_PROTO_ICMPV6 capability enabled.

By default Linux bridge is created with multicast enabled which can be
modified by 'mcast_snooping' argument:

$ ip link set name my_bridge type bridge mcast_snooping 0

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>