Fix PR221990 - Assertion at iflib.c:1947

ifl_pidx and ifl_credits are going out of sync in _iflib_fl_refill() as they
use different update log. Use the same update logic for both, and add a
final call to isc_rxd_refill() to handle early exits from the loop.

PR: 221990
Reported by: pho
Reviewed by: sbruno
Approved by: sbruno (mentor)
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12798