iflib: refactor netmap_fl_refill and fix off-by-one issue

First, fix the initialization of the fl->ifl_rxd_idxs array,
which was affected by an off-by-one bug.
Once there, refactor the function to use better names for
local variables, optimize the variable assignments, and
merge the bus_dmamap_sync() inner loop with the outer one.

PR: 248494
MFC after: 3 weeks