sfxge(4): ensure EvQ poll stops when abort is requested

[arybchik]

Summary

If an event handler requested an abort, only the inner loop was
guarenteed to be broken out of - the outer loop could continue
if total == batch.

Fix this by poisoning batch to ensure it is different to total.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine