Revert "genetlink: hold RCU in genlmsg_mcast()"

This reverts commit 0d2bd44b which is
commit 56440d7ec28d60f8da3bfa09062b3368ff9b16db 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: Ie8c829833eef0ab6b68677f3da08620e894787a0
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>