[PATCH] loop: potential kernel hang waiting for kthread

Check that kernel_thread() succeeded, so we don't wait for something which
cannot happen.

Signed-off-by: Herbert Poetzl <herbert@13thfloor.at>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>