sfc: extend the locking on mcdi->seqno

seqno could be read as a stale value outside of the lock. The lock is
already acquired to protect the modification of seqno against a possible
race condition. Place the reading of this value also inside this locking
to protect it against a possible race condition.
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Acked-by: Martin Habets <habetsm.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/mcdi.c

@@ -163,9 +163,9 @@ static void efx_mcdi_send_request(struct efx_nic *efx, unsigned cmd,
 	/* Serialise with efx_mcdi_ev_cpl() and efx_mcdi_ev_death() */
 	spin_lock_bh(&mcdi->iface_lock);
 	++mcdi->seqno;
+	seqno = mcdi->seqno & SEQ_MASK;
 	spin_unlock_bh(&mcdi->iface_lock);
 
-	seqno = mcdi->seqno & SEQ_MASK;
 	xflags = 0;
 	if (mcdi->mode == MCDI_MODE_EVENTS)
 		xflags |= MCDI_HEADER_XFLAGS_EVREQ;