riscv: select ARCH_USE_CMPXCHG_LOCKREF

Select ARCH_USE_CMPXCHG_LOCKREF to enable the cmpxchg-based lockless lockref
implementation for riscv.

Using Linus' test case[1] on TH1520 platform, I see a 11.2% improvement.
On JH7110 platform, I see 12.0% improvement.

Link: http://marc.info/?l=linux-fsdevel&m=137782380714721&w=4 [1]
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>

arch/riscv/Kconfig

@@ -35,6 +35,7 @@ config RISCV
 	select ARCH_KEEP_MEMBLOCK
 	select ARCH_OPTIONAL_KERNEL_RWX if ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_OPTIONAL_KERNEL_RWX_DEFAULT
+	select ARCH_USE_CMPXCHG_LOCKREF
 	select ARCH_USE_QUEUED_SPINLOCKS
 	select ARCH_USE_QUEUED_RWLOCKS
 	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU