Fix devstat on md devices, second attempt. r341765 depends on
g_io_deliver() finishing initialization of the bio, but g_io_deliver()
actually destroys the bio. INVARIANTS makes the bug obvious by
overwriting the bio with garbage.

Restore the old order for calling devstat (except don't restore not calling
it for the error case), and translate to the devstat KPI so that this order
works.

Reviewed by: kib