genetlink: Remove smp_read_barrier_depends() from comment

Now that smp_read_barrier_depends() has been de-emphasized, the less
said about it, the better.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>