sh_eth: Fix serialisation of interrupt disable with interrupt & NAPI handlers

In order to stop the RX path accessing the RX ring while it's being
stopped or resized, we clear the interrupt mask (EESIPR) and then call
free_irq() or synchronise_irq().  This is insufficient because the
interrupt handler or NAPI poller may set EESIPR again after we clear
it.  Also, in sh_eth_set_ringparam() we currently don't disable NAPI
polling at all.

I could easily trigger a crash by running the loop:

   while ethtool -G eth0 rx 128 && ethtool -G eth0 rx 64; do echo -n .; done

and 'ping -f' toward the sh_eth port from another machine.

To fix this:
- Add a software flag (irq_enabled) to signal whether interrupts
  should be enabled
- In the interrupt handler, if the flag is clear then clear EESIPR
  and return
- In the NAPI poller, if the flag is clear then don't set EESIPR
- Set the flag before enabling interrupts in sh_eth_dev_init() and
  sh_eth_set_ringparam()
- Clear the flag and serialise with the interrupt and NAPI
  handlers before clearing EESIPR in sh_eth_close() and
  sh_eth_set_ringparam()

After this, I could run the loop for 100,000 iterations successfully.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/sh_eth.h

@@ -513,6 +513,7 @@ struct sh_eth_private {
 	u32 rx_buf_sz;			/* Based on MTU+slack. */
 	int edmac_endian;
 	struct napi_struct napi;
+	bool irq_enabled;
 	/* MII transceiver section. */
 	u32 phy_id;			/* PHY ID */
 	struct mii_bus *mii_bus;	/* MDIO bus control */