Revert "genetlink: hold RCU in genlmsg_mcast()"

This reverts commit ae53d09f which is
commit 56440d7e upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I62229b26a4fd7dd4141a0342e3b7298ed3ee7942
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>