net: bridge: mcast: don't ignore return value of __grp_src_toex_excl

When we're handling TO_EXCLUDE report in EXCLUDE filter mode we should
not ignore the return value of __grp_src_toex_excl() as we'll miss
sending notifications about group changes.

Fixes: 5bf1e00b

 ("net: bridge: mcast: support for IGMPV3/MLDv2 CHANGE_TO_INCLUDE/EXCLUDE report")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>