tools/lib/lockdep: Add userspace version of READ_ONCE()

This was added to the kernel code in <1658d35e> ("list: Use
READ_ONCE() when testing for empty lists").

There's nothing special we need to do about it in userspace.

Signed-off-by: Alfredo Alvarez Fernandez <alfredoalvarezfernandez@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1455864533-7536-2-git-send-email-alfredoalvarezernandez@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>