nds32: use get_kernel_nofault in dump_mem

Use the proper get_kernel_nofault helper to access an unsafe kernel
pointer without faulting instead of playing with set_fs and get_user.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Nick Hu <nickhu@andestech.com>
Acked-by: Greentime Hu <green.hu@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>