The first packet after the ring is initialized was never
completed as isc_txd_credits_update() would not include it in the
count of completed packets. This caused netmap to never complete
a batch. See PR 233022 for more details.

PR:		233022
Reported by:	lev
Reviewed by:	lev
MFC after:	3 days
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D17931