pfsync: add missing unlock in pfsync_defer_tmo()

The callout for pfsync_defer_tmo() is created with
CALLOUT_RETURNUNLOCKED, because while the callout framework takes care
of taking the lock we want to run a few operations outside of the lock,
so we unlock ourselves.

However, if `sc->sc_sync_if == NULL` we return without releasing the
lock, and leak the lock, causing later deadlocks.
Ensure we always release the bucket lock when we exit pfsync_defer_tmo()

PR:		268246
MFC after:	1 week
Sponsored by:	Rubicon Communications, LLC (Netgate)

(cherry picked from commit 844ad28)

sys/netpfil/pf/if_pfsync.c

@@ -1830,8 +1830,10 @@ pfsync_defer_tmo(void *arg)
 
 	PFSYNC_BUCKET_LOCK_ASSERT(b);
 
-	if (sc->sc_sync_if == NULL)
+	if (sc->sc_sync_if == NULL) {
+		PFSYNC_BUCKET_UNLOCK(b);
 		return;
+	}
 
 	NET_EPOCH_ENTER(et);
 	CURVNET_SET(sc->sc_sync_if->if_vnet);