kmemleak: add scheduling point to kmemleak_scan()

kmemleak_scan() will scan struct page for each node and it can be really
large and resulting in a soft lockup.  We have seen a soft lockup when
do scan while compile kernel:

  watchdog: BUG: soft lockup - CPU#53 stuck for 22s! [bash:10287]
 [...]
  Call Trace:
   kmemleak_scan+0x21a/0x4c0
   kmemleak_write+0x312/0x350
   full_proxy_write+0x5a/0xa0
   __vfs_write+0x33/0x150
   vfs_write+0xad/0x1a0
   SyS_write+0x52/0xc0
   do_syscall_64+0x61/0x1a0
   entry_SYSCALL64_slow_path+0x25/0x25

Fix this by adding cond_resched every MAX_SCAN_SIZE.

Link: http://lkml.kernel.org/r/1511439788-20099-1-git-send-email-xieyisheng1@huawei.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kmemleak.c

@@ -1523,6 +1523,8 @@ static void kmemleak_scan(void)
 			if (page_count(page) == 0)
 				continue;
 			scan_block(page, page + 1, NULL);
+			if (!(pfn % (MAX_SCAN_SIZE / sizeof(*page))))
+				cond_resched();
 		}
 	}
 	put_online_mems();