ext4: fix journal ref count in move_extent_par_page

move_extent_par_page calls a_ops->write_begin() to increase journal
handler's reference count. However, if either mext_replace_branches()
or ext4_get_block fails, the increased reference count isn't
decreased. This will cause a later attempt to umount of the fs to hang
forever. The patch addresses the issue by calling ext4_journal_stop()
if page is not NULL (which means a_ops->write_end() isn't invoked).
Signed-off-by: NPeng Tao <bergwolf@gmail.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/move_extent.c

@@ -871,6 +871,7 @@ move_extent_par_page(struct file *o_filp, struct inode *donor_inode,
 		if (PageLocked(page))
 			unlock_page(page);
 		page_cache_release(page);
+		ext4_journal_stop(handle);
 	}
 out2:
 	ext4_journal_stop(handle);