ext4: don't leave PageWriteback set after memory failure

In ext4_bio_write_page(), if the memory allocation for the struct
ext4_io_page fails, it returns with the page's PageWriteback flag set.
This will end up causing the page not to skip writeback in
WB_SYNC_NONE mode, and in WB_SYNC_ALL mode (i.e., on a sync, fsync, or
umount) the writeback daemon will get stuck forever on the
wait_on_page_writeback() function in write_cache_pages_da().

Or, if journalling is enabled and the file gets deleted, it the
journal thread can get stuck in journal_finish_inode_data_buffers()
call to filemap_fdatawait().

Another place where things can get hung up is in
truncate_inode_pages(), called out of ext4_evict_inode().

Fix this by not setting PageWriteback until after we have successfully
allocated the struct ext4_io_page.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/page-io.c

@@ -381,8 +381,6 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
 
 	BUG_ON(!PageLocked(page));
 	BUG_ON(PageWriteback(page));
-	set_page_writeback(page);
-	ClearPageError(page);
 
 	io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
 	if (!io_page) {
@@ -393,6 +391,8 @@ int ext4_bio_write_page(struct ext4_io_submit *io,
 	io_page->p_page = page;
 	atomic_set(&io_page->p_count, 1);
 	get_page(page);
+	set_page_writeback(page);
+	ClearPageError(page);
 
 	for (bh = head = page_buffers(page), block_start = 0;
 	     bh != head || !block_start;