riscv: mm: Use PMD_SIZE as map size for linear mapping

To avoid pagefault when machine_kexec() call kexec_method
that is control_code_buffer.

When uses PUD_SIZE as map size, __set_memory only set init_mm
So it casues other task page fault when uses the pud entry
modified by init_mm.

Signed-off-by: Xiaoguang Xing <xiaoguang.xing@sophgo.com>