RISC-V: Avoid empty create_*_mapping definitions

These trigger a handful of build warnings.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 677b9eb ("riscv: mm: Prepare pt_ops helper functions for sv57")
Link: https://lore.kernel.org/r/20220420184056.7886-2-palmer@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/init.c

@@ -567,9 +567,9 @@ static void __init create_p4d_mapping(p4d_t *p4dp,
 	create_pte_mapping(__nextp, __va, __pa, __sz, __prot)
 #define fixmap_pgd_next		((uintptr_t)fixmap_pte)
 #define early_dtb_pgd_next	((uintptr_t)early_dtb_pmd)
-#define create_p4d_mapping(__pmdp, __va, __pa, __sz, __prot)
-#define create_pud_mapping(__pmdp, __va, __pa, __sz, __prot)
-#define create_pmd_mapping(__pmdp, __va, __pa, __sz, __prot)
+#define create_p4d_mapping(__pmdp, __va, __pa, __sz, __prot) do {} while(0)
+#define create_pud_mapping(__pmdp, __va, __pa, __sz, __prot) do {} while(0)
+#define create_pmd_mapping(__pmdp, __va, __pa, __sz, __prot) do {} while(0)
 #endif /* __PAGETABLE_PMD_FOLDED */
 
 void __init create_pgd_mapping(pgd_t *pgdp,