iflib: fix book keeping

iflib_txq_drain() returns the number of consumed entries. In the case
of TSO, a single entry can contain multiple TCP packets.

Reported by: Ricardo Branco, David Wolfskill
Reviewed by: gallatin
Fixes: 3fade68cfdf9 ("iflib: accurately count bytes/segments for TSO")
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D56509