riscv: enable HAVE_FAST_GUP if MMU

Activate the fast gup for riscv mmu platforms. Here are some
GUP_FAST_BENCHMARK performance numbers:

Before the patch:
GUP_FAST_BENCHMARK: Time: get:53203 put:5085 us

After the patch:
GUP_FAST_BENCHMARK: Time: get:17711 put:5060 us

The get time is reduced by 66.7%! IOW, 3x get speed!

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>

arch/riscv/Kconfig

@@ -120,6 +120,7 @@ config RISCV
 	select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
 	select HAVE_EBPF_JIT if MMU
+	select HAVE_FAST_GUP if MMU
 	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_GCC_PLUGINS

arch/riscv/include/asm/pgtable.h

@@ -672,6 +672,12 @@ static inline int pmd_write(pmd_t pmd)
 	return pte_write(pmd_pte(pmd));
 }
 
+#define pud_write pud_write
+static inline int pud_write(pud_t pud)
+{
+	return pte_write(pud_pte(pud));
+}
+
 static inline int pmd_dirty(pmd_t pmd)
 {
 	return pte_dirty(pmd_pte(pmd));