bridge: mdb: print source list when available

bridge: mdb: print source list when available

Print the mdb entry's source list when it's available if the user
requested to show details (-d). Each source has an associated timer
which controls if traffic should be forwarded to that S,G entry (if the
timer is non-zero traffic is forwarded, otherwise it's not).
Currently the source list is kernel controlled and can't be changed by
user-space.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David Ahern <dsahern@gmail.com>