Revert "genetlink: hold RCU in genlmsg_mcast()"

This reverts commit 03ca631c 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: I8e2c18c063388d090dfb5a882c4c80a8c4702122
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>