Add an EXPORT_SYMBOL() to genl_unregister_mc_group(), to allow
unregistering groups on the run. EXPORT_SYMBOL_GPL() is not used as
the rest of the functions exported by this module (eg:
genl_register_mc_group) are also not _GPL().

Cleanup is currently done when unregistering a family, but there is
no way to unregister a single multicast group due to that function not
being exported. Seems to be a mistake as it is documented as for
external consumption.

This is needed by the WiMAX stack to be able to cleanup unused mc
groups.
Signed-off-by: NInaky Perez-Gonzalez <inaky@linux.intel.com>
Acked-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>