MFC r364655

iflib: fix isc_rxd_flush call in netmap_fl_refill()

The semantic of the pidx argument of isc_rxd_flush() is the
last valid index of in the free list, rather than the next
index to be published. However, netmap was still using the
old convention. While there, also refactor the netmap_fl_refill()
to simplify a little bit and add an assertion.