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

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
Differential Revision:  https://reviews.freebsd.org/D18287

sys/dev/sfxge/common/efx_ev.c

@@ -509,6 +509,14 @@ efx_ev_qpoll(
 			if (should_abort) {
 				/* Ignore subsequent events */
 				total = index + 1;
+
+				/*
+				 * Poison batch to ensure the outer
+				 * loop is broken out of.
+				 */
+				EFSYS_ASSERT(batch <= EFX_EV_BATCH);
+				batch += (EFX_EV_BATCH << 1);
+				EFSYS_ASSERT(total != batch);
 				break;
 			}
 		}