vmstat: fix offset calculation on void*

Fix regression introduced by commit 79da826aee6 ("writeback: report
dirty thresholds in /proc/vmstat").

The incorrect pointer arithmetic can result in problems like this:

BUG: unable to handle kernel paging request at 07c06d16
IP: [<c050c336>] strnlen+0x6/0x20
Call Trace:
[<c050a249>] ? string+0x39/0xe0
[<c042be6b>] ? __wake_up_common+0x4b/0x80
[<c050afcc>] ? vsnprintf+0x1ec/0x380
[<c04b380e>] ? seq_printf+0x2e/0x60
[<c04829a6>] ? vmstat_show+0x26/0x30
[<c04b3bb6>] ? seq_read+0xa6/0x380
[<c04b3b10>] ? seq_read+0x0/0x380
[<c04d5d2f>] ? proc_reg_read+0x5f/0x90
[<c049c4a1>] ? vfs_read+0xa1/0x140
[<c04d5cd0>] ? proc_reg_read+0x0/0x90
[<c049c981>] ? sys_read+0x41/0x70
[<c0402bd0>] ? sysenter_do_call+0x12/0x26

Reported-by: Tetsuo Handa <[e-mail]>
Cc: Michael Rubin <[e-mail]>
Signed-off-by: Wu Fengguang <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmstat.c

···
 	v[PGPGIN] /= 2;		/* sectors -> kbytes */
 	v[PGPGOUT] /= 2;
 #endif
-	return m->private + *pos;
+	return (unsigned long *)m->private + *pos;
 }
 
 static void *vmstat_next(struct seq_file *m, void *arg, loff_t *pos)