if_ntb: Reuse receive buffers correctly

Discard the unused rx_free_q. Instead, reuse inputed packets by putting
them back on the *pend* queue after reinitialization.

If tx or rx handlers are unavailable, free mbufs rather than leaking
them.

With this change, if_ntb can receive more than 100
(NTB_QP_DEF_NUM_ENTRIES) packets.

Sponsored by: EMC / Isilon Storage Division