[MIPS] sparsemem: fix crash in show_mem

With sparsemem, pfn should be checked by pfn_valid() before pfn_to_page().

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
1 file changed