x86/mm/ptdump: Fix soft lockup in page table walker

CONFIG_KASAN=y needs a lot of virtual memory mapped for its shadow.
In that case ptdump_walk_pgd_level_core() takes a lot of time to
walk across all page tables and doing this without
a rescheduling causes soft lockups:

NMI watchdog: BUG: soft lockup - CPU#3 stuck for 23s! [swapper/0:1]
...
Call Trace:
ptdump_walk_pgd_level_core+0x40c/0x550
ptdump_walk_pgd_level_checkwx+0x17/0x20
mark_rodata_ro+0x13b/0x150
kernel_init+0x2f/0x120
ret_from_fork+0x2c/0x40

I guess that this issue might arise even without KASAN on huge machines
with several terabytes of RAM.

Stick cond_resched() in pgd loop to fix this.

Reported-by: Tobias Regnery <[e-mail]>
Signed-off-by: Andrey Ryabinin <[e-mail]>
Cc: [e-mail]
Cc: Alexander Potapenko <[e-mail]>
Cc: "Paul E . McKenney" <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

arch/x86/mm/dump_pagetables.c

···
 #include <linux/debugfs.h>
 #include <linux/mm.h>
 #include <linux/init.h>
+#include <linux/sched.h>
 #include <linux/seq_file.h>
 
 #include <asm/pgtable.h>
···
 		} else
 			note_page(m, &st, __pgprot(0), 1);
 
+		cond_resched();
 		start++;
 	}