atomics/treewide: Make conditional inc/dec ops optional

The conditional inc/dec ops differ for atomic_t and atomic64_t:

- atomic_inc_unless_positive() is optional for atomic_t, and doesn't exist for atomic64_t.
- atomic_dec_unless_negative() is optional for atomic_t, and doesn't exist for atomic64_t.
- atomic_dec_if_positive is optional for atomic_t, and is mandatory for atomic64_t.

Let's make these consistently optional for both. At the same time, let's
clean up the existing fallbacks to use atomic_try_cmpxchg().

The instrumented atomics are updated accordingly.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20180621121321.4761-18-mark.rutland@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/powerpc/include/asm/atomic.h

@@ -488,6 +488,7 @@ static __inline__ long atomic64_dec_if_positive(atomic64_t *v)
 
 	return t;
 }
+#define atomic64_dec_if_positive atomic64_dec_if_positive
 
 #define atomic64_cmpxchg(v, o, n) (cmpxchg(&((v)->counter), (o), (n)))
 #define atomic64_cmpxchg_relaxed(v, o, n) \