riscv: Fix build error if !CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION

The new riscv specific arch_hugetlb_migration_supported() must be
guarded with a #ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION to avoid
the following build error:

In file included from include/linux/hugetlb.h:851,
                    from kernel/fork.c:52:
>> arch/riscv/include/asm/hugetlb.h:15:42: error: static declaration of 'arch_hugetlb_migration_supported' follows non-static declaration
      15 | #define arch_hugetlb_migration_supported arch_hugetlb_migration_supported
         |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/hugetlb.h:916:20: note: in expansion of macro 'arch_hugetlb_migration_supported'
     916 | static inline bool arch_hugetlb_migration_supported(struct hstate *h)
         |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   arch/riscv/include/asm/hugetlb.h:14:6: note: previous declaration of 'arch_hugetlb_migration_supported' with type 'bool(struct hstate *)' {aka '_Bool(struct hstate *)'}
      14 | bool arch_hugetlb_migration_supported(struct hstate *h);
         |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402110258.CV51JlEI-lkp@intel.com/
Fixes: ce68c035 ("riscv: Fix arch_hugetlb_migration_supported() for NAPOT")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240211083640.756583-1-alexghiti@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/include/asm/hugetlb.h

@@ -11,8 +11,10 @@ static inline void arch_clear_hugepage_flags(struct page *page)
 }
 #define arch_clear_hugepage_flags arch_clear_hugepage_flags
 
+#ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
 bool arch_hugetlb_migration_supported(struct hstate *h);
 #define arch_hugetlb_migration_supported arch_hugetlb_migration_supported
+#endif
 
 #ifdef CONFIG_RISCV_ISA_SVNAPOT
 #define __HAVE_ARCH_HUGE_PTE_CLEAR

arch/riscv/mm/hugetlbpage.c

@@ -426,10 +426,12 @@ bool __init arch_hugetlb_valid_size(unsigned long size)
 	return __hugetlb_valid_size(size);
 }
 
+#ifdef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
 bool arch_hugetlb_migration_supported(struct hstate *h)
 {
 	return __hugetlb_valid_size(huge_page_size(h));
 }
+#endif
 
 #ifdef CONFIG_CONTIG_ALLOC
 static __init int gigantic_pages_init(void)