bridge: Immediately drop interfaces that can't be opened.

bridge: Immediately drop interfaces that can't be opened.

Previously we would keep interfaces around that couldn't be opened
because they might be internal interfaces that are created later.
However, this leads to a race condition if the interface appears
after we try to create it and fails since some operations may
succeed.  Instead, give up on the interface immediately if it can't
be opened and isn't internal (which we control and so won't have
this issue).

Bug #2737