net: mscc: ocelot: fix missing unlock on error in ocelot_hwstamp_set()

Add the missing mutex_unlock before return from function
ocelot_hwstamp_set() in the ocelot_setup_ptp_traps() error
handling case.

Fixes: 96ca08c0 ("net: mscc: ocelot: set up traps for PTP packets")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211129151652.1165433-1-weiyongjun1@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mscc/ocelot.c

@@ -1563,8 +1563,10 @@ int ocelot_hwstamp_set(struct ocelot *ocelot, int port, struct ifreq *ifr)
 	}
 
 	err = ocelot_setup_ptp_traps(ocelot, port, l2, l4);
-	if (err)
+	if (err) {
+		mutex_unlock(&ocelot->ptp_lock);
 		return err;
+	}
 
 	if (l2 && l4)
 		cfg.rx_filter = HWTSTAMP_FILTER_PTP_V2_EVENT;