[MCAST]: ip[6]_mc_add_src should be called when number of sources is zero

And filter mode is exclude.

Further explanation by David Stevens:

Multicast source filters aren't widely used yet, and that's really the only
feature that's affected if an application actually exercises this bug, as far
as I can tell. An ordinary filter-less multicast join should still work, and
only forwarded multicast traffic making use of filters and doing empty-source
filters with the MSFILTER ioctl would be at risk of not getting multicast
traffic forwarded to them because the reports generated would not be based on
the correct counts.

Signed-off-by: Yan Zheng <yanzheng@21cn.com>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>