parisc: implement irq stacks - part 2 (v2)

This patch fixes few build issues which were introduced with the last
irq stack patch, e.g. the combination of stack overflow check and irq
stack.

Furthermore we now do proper locking and change the irq bh handler
to use the irq stack as well.

In /proc/interrupts one now can monitor how huge the irq stack has grown
and how often it was preferred over the kernel stack.

IRQ stacks are now enabled by default just to make sure that we not
overflow the kernel stack by accident.

Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/Kconfig

@@ -245,7 +245,7 @@ config SMP
 
 config IRQSTACKS
 	bool "Use separate kernel stacks when processing interrupts"
-	default n
+	default y
 	help
 	  If you say Y here the kernel will use separate kernel stacks
 	  for handling hard and soft interrupts.  This can help avoid