riscv: atomic: Add custom conditional atomic operation implementation

Add conditional atomic operations' custom implementation (similar
to dec_if_positive), here is the list:
 - arch_atomic_inc_unless_negative
 - arch_atomic_dec_unless_positive
 - arch_atomic64_inc_unless_negative
 - arch_atomic64_dec_unless_positive

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220505035526.2974382-4-guoren@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>