Btrfs: fix panic on error during mount

This needs to be applied on top of my previous patches, but is needed for more
than just my new stuff.  We're going to the wrong label when we have an error,
we try to stop the workers, but they are started below all of this code.  This
fixes it so we go to the right error label and not panic when we fail one of
these cases.

Signed-off-by: Josef Bacik <jbacik@redhat.com>