Revert "genetlink: hold RCU in genlmsg_mcast()"

This reverts commit 4af714e8 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: I812d261d139e7f73a135c3445b7733b7611c23e0
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>