ext4: fix setting random pages PageUptodate

ext4_end_bio calls put_page and kmem_cache_free before calling
SetPageUpdate(). This can result in setting the PageUptodate bit on
random pages and causes the following BUG:

BUG: Bad page state in process rm pfn:52e54
page:ffffea0001222260 count:0 mapcount:0 mapping: (null) index:0x0
arch kernel: page flags: 0x4000000000000008(uptodate)

Fix the problem by moving put_io_page() after the SetPageUpdate() call.

Thanks to Hugh Dickins for analyzing this problem.

Reported-by: Markus Trippelsdorf <[e-mail]>
Tested-by: Markus Trippelsdorf <[e-mail]>
Signed-off-by: Markus Trippelsdorf <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/page-io.c

···
 			} while (bh != head);
 		}
 
-		put_io_page(io_end->pages[i]);
-
 		/*
 		 * If this is a partial write which happened to make
 		 * all buffers uptodate then we can optimize away a
···
 		 */
 		if (!partial_write)
 			SetPageUptodate(page);
+
+		put_io_page(io_end->pages[i]);
 	}
 	io_end->num_io_pages = 0;
 	inode = io_end->inode;