• In _iflib_fl_refill(), don't mark an RX buffer as available in the corresponding bitmap before adding an mbuf has actually succeeded. Previously, m_gethdr(M_NOWAIT, ...) failing caused a "hole" in the RX ring but not in its bitmap. One implication of such a hole was that in a subsequent call to _iflib_fl_refill() with the RX buffer accounting still indicating another reclaimable buffer, bit_ffc(3) nevertheless returned -1 in frag_idx which in turn caused havoc when used as an index. Thus, additionally assert that frag_idx is 0 or greater. Another possible consequence of a hole in the RX ring was a NULL- dereference when trying to use the unallocated mbuf, for example in iflib_rxd_pkt_get().
  
  While at it, make the variable declarations in _iflib_fl_refill() conform to style(9) and remove redundant checks already performed by bit_ffc{,_at}(3).

• In iflib_queues_alloc(), don't pass redundant M_ZERO to bit_alloc(3).

Reported and tested by: pho