module: freebsd: avoid a taking a destroyed lock in zfs_zevent bits

At shutdown time, we drain all of the zevents and set the
ZEVENT_SHUTDOWN flag.  On FreeBSD, we may end up calling
zfs_zevent_destroy() after the zevent_lock has been destroyed while
the sysevent thread is winding down; we observe ESHUTDOWN, then back
out.

Events have already been drained, so just inline the kmem_free call in
sysevent_worker() to avoid the race, and document the assumption that
zfs_zevent_destroy doesn't do anything else useful at that point.

This fixes a panic that can occur at module unload time.

Signed-off-by: Kyle Evans <kevans@FreeBSD.org>

module/os/freebsd/spl/spl_sysevent.c

@@ -250,7 +250,17 @@ sysevent_worker(void *arg __unused)
 			nvlist_free(event);
 		}
 	}
-	zfs_zevent_destroy(ze);
+
+	/*
+	 * We avoid zfs_zevent_destroy() here because we're otherwise racing
+	 * against fm_fini() destroying the zevent_lock.  zfs_zevent_destroy()
+	 * will currently only clear `ze->ze_zevent` from an event list then
+	 * free `ze`, so just inline the free() here -- events have already
+	 * been drained.
+	 */
+	VERIFY3P(ze->ze_zevent, ==, NULL);
+	kmem_free(ze, sizeof (zfs_zevent_t));
+
 	kthread_exit();
 }