bio_copy is doing

	vfrom = kmap_atomic(bv->bv_page, KM_BIO_IRQ);
	vto = kmap_atomic(bbv->bv_page, KM_BIO_IRQ);

which, if I understand atomic kmaps, is incorrect.  Both source and
dest will get the same pte.

The patch creates a separate atomic kmap member for the destination and
source of this copy.