locking/lockref: Remove homebrew cmpxchg64_relaxed() macro definition

cmpxchg64_relaxed() is now defined by linux/atomic.h, so we can
remove our local definition from the lockref code.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hp.com
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1438880084-18856-5-git-send-email-will.deacon@arm.com


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