Use try_cmpxchg64 instead of cmpxchg64 in CMPXCHG_LOOP macro.
x86 CMPXCHG instruction returns success in ZF flag, so this
change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg). The main loop of lockref_get improves from:

  13:	48 89 c1             	mov    %rax,%rcx
  16:	48 c1 f9 20          	sar    $0x20,%rcx
  1a:	83 c1 01             	add    $0x1,%ecx
  1d:	48 89 ce             	mov    %rcx,%rsi
  20:	89 c1                	mov    %eax,%ecx
  22:	48 89 d0             	mov    %rdx,%rax
  25:	48 c1 e6 20          	shl    $0x20,%rsi
  29:	48 09 f1             	or     %rsi,%rcx
  2c:	f0 48 0f b1 4d 00    	lock cmpxchg %rcx,0x0(%rbp)
  32:	48 39 d0             	cmp    %rdx,%rax
  35:	75 17                	jne    4e <lockref_get+0x4e>

to:

  13:	48 89 ca             	mov    %rcx,%rdx
  16:	48 c1 fa 20          	sar    $0x20,%rdx
  1a:	83 c2 01             	add    $0x1,%edx
  1d:	48 89 d6             	mov    %rdx,%rsi
  20:	89 ca                	mov    %ecx,%edx
  22:	48 c1 e6 20          	shl    $0x20,%rsi
  26:	48 09 f2             	or     %rsi,%rdx
  29:	f0 48 0f b1 55 00    	lock cmpxchg %rdx,0x0(%rbp)
  2f:	75 02                	jne    33 <lockref_get+0x33>

[ Michael Ellerman and Mark Rutland confirm that code generation on
  powerpc and arm64 respectively is also ok, even though they do not
  have a native arch_try_cmpxchg() implementation, and rely on the
  default fallback case    - Linus ]
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hp.com
Cc: paulmck@linux.vnet.ibm.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>