MIPS: atomic: Use _atomic barriers in atomic_sub_if_positive()

Use smp_mb__before_atomic() & smp_mb__after_atomic() in
atomic_sub_if_positive() rather than the equivalent
smp_mb__before_llsc() & smp_llsc_mb(). The former are more standard &
this preps us for avoiding redundant duplicate barriers on Loongson3 in
a later patch.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: linux-kernel@vger.kernel.org