gve: Fix qpl_buf_head being initialized improperly

Currently, for DQO QPL our MPASS assertion on qpl_buf_head for available
pending_pkts (i.e. not holding a packet) fails due to incorrect
initialization. The MPASS fails on the first run of packets through the
ring when INVARIANTS is on, and when INVARIANTS is off, things work
without a bug.

The MPASS guards against improper reaping of "pending_pkt" objects,
and thus was failing for the first run through the ring. By correctly
initializing the objects in this patch we make the MPASS not fail on the
first run too.

Signed-off-by: Vee Agarwal <veethebee@google.com>
Signed-off-by: Jasper Tran O'Leary <jtranoleary@google.com>

Reviewed by: delphij, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48968

(cherry picked from commit 73c3fe4db3ebc2bd6cb732aae77ea017fd376d22)