The change adopts the idea of fxp to drop the incoming packet and panic
if the old mbuf cannot be reloaded. Since the bus_dmamap is allocated
during attach, this is not supposed to happen. Since a lot of code moves
anyway, factor out the allocation of RX ring elements, which is shared
between the init path and the RX interrupt path.
XXX A better fix might be to borrow the mbuf from the logic end of the
XXX ring buffer, but that needs more involved driver changes.
Reviewed by dyoung@ and nick@
35b726a534