riscv: mm: pmdp_huge_get_and_clear(): avoid atomic ops when !CONFIG_SMP

When !CONFIG_SMP, there's no need for atomic operations in
pmdp_huge_get_and_clear(), so, similar to what x86 does, let's not use
atomics in this case.  See also commit 546e42c ("riscv: Use an
atomic xchg in pudp_huge_get_and_clear()").

Cc: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Paul Walmsley <pjw@kernel.org>

Author: Paul Walmsley

arch/riscv/include/asm/pgtable.h

@@ -997,7 +997,13 @@ static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
 static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
 					unsigned long address, pmd_t *pmdp)
 {
+#ifdef CONFIG_SMP
 	pmd_t pmd = __pmd(atomic_long_xchg((atomic_long_t *)pmdp, 0));
+#else
+	pmd_t pmd = *pmdp;
+
+	pmd_clear(pmdp);
+#endif
 
 	page_table_check_pmd_clear(mm, pmd);