stmmac: Don't access tx_q->dirty_tx before netif_tx_lock

This is the possible reason for different hard to reproduce
problems on my ARMv7-SMP test system.

The symptoms are in recent kernels imprecise external aborts,
and in older kernels various kinds of network stalls and
unexpected page allocation failures.

My testing indicates that the trouble started between v4.5 and v4.6
and prevails up to v4.14.

Using the dirty_tx before acquiring the spin lock is clearly
wrong and was first introduced with v4.6.

Fixes: e3ad57c9 ("stmmac: review RX/TX ring management")

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Signed-off-by: David S. Miller <davem@davemloft.net>