s390/mm: add cond_resched call to kernel page table dumper

Walking kernel page tables within the kernel page table dumper may
potentially take a lot of time. This may lead to soft lockup warning
messages.
To avoid this add a cond_resched call for each pgd_level iteration.

This is the same as "x86/mm/ptdump: Fix soft lockup in page table
walker" for x86.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/dump_pagetables.c

 #include <linux/seq_file.h>
 #include <linux/debugfs.h>
+#include <linux/sched.h>
 #include <linux/mm.h>
 #include <asm/sections.h>
 #include <asm/pgtable.h>
@@ -187,6 +188,7 @@ static void walk_pgd_level(struct seq_file *m)
 		else
 			note_page(m, &st, _PAGE_INVALID, 1);
 		addr += PGDIR_SIZE;
+		cond_resched();
 	}
 	/* Flush out the last page */
 	st.current_address = max_addr;