Revert "genetlink: disallow subscribing to unknown mcast groups"

Commit 5ad63005 ("genetlink: disallow subscribing to unknown
mcast groups") disallows userspace to subscribe to groups that don't
exist in kernel. As a result, communication between processes is
not possible unless they explicitly register a dummy group with the
kernel even if the communication is between userspace processes
only. NETLINK_USERSOCK cannot be used here since userspace processes
would require CAP_NET_ADMIN to receive multicast messages which
is available for priveleged processes only.

Fix this problem by reverting the change till a solution is
determined internally and upstream discussion.

CRs-Fixed: 1078373
Change-Id: Id559d9ef9d1e0a25e3bbdc81503978f01c6ed85f
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>