riscv: Fix arch_hugetlb_migration_supported() for NAPOT

arch_hugetlb_migration_supported() must be reimplemented to add support
for NAPOT hugepages, which is done here.

Fixes: 82a1a1f3

 ("riscv: mm: support Svnapot in hugetlb page")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240130120114.106003-1-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>