Before trying to update metadata (so open consumer for writing), be sure

Before trying to update metadata (so open consumer for writing), be sure
that the events queue is empty. In other case we're able to hit the race
where for example da0s1 is tasted by some other class, which means that
da0 is open with exclusive bit set, which means that we can't open da0
for writing if it is our component.

Reported by:	Attila Nagy <bra@fsn.hu> (and somebody else sometime ago,
		                          but I cannot find who it was)