Btrfs: don't wait on ordered extents if we have a trans open

Dave was hitting a lockdep warning because we're now properly taking the ordered
operations mutex in the ordered wait stuff.  This is because some cases we will
have a trans handle when we are flushing delalloc space, but we can't wait on
ordered extents because we could potentially deadlock, so fix this by not doing
the wait if we have a trans handle.  Thanks

Reported-and-tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>