atomics/arm: Define atomic64_fetch_add_unless()

As a step towards unifying the atomic/atomic64/atomic_long APIs, this
patch converts the arch/arm implementation of atomic64_add_unless() into
an implementation of atomic64_fetch_add_unless().

A wrapper in <linux/atomic.h> will build atomic_add_unless() atop of
this, provided it is given a preprocessor definition.

No functional change is intended 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: Russell King <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20180621121321.4761-12-mark.rutland@arm.com


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