riscv: mm: call best_map_size many times during linear-mapping

Modify the best_map_size function to give map_size many times instead
of only once, so a memory region can be mapped by both PMD_SIZE and
PAGE_SIZE.

Signed-off-by: Qinglin Pan <panqinglin2020@iscas.ac.cn>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20221128023643.329091-1-panqinglin2020@iscas.ac.cn


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>