jbd: check for error returned by kthread_create on creating journal thread

If the thread failed to create the subsequent wait_event will hang forever.

This is likely to happen if kernel hits max_threads limit.

Will be critical for virtualization systems that limit the number of tasks
and kernel memory usage within the container.

(akpm: JBD should be converted fully to the kthread API: kthread_should_stop()
and kthread_stop()).

Cc: <linux-ext4@vger.kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>