irq: Better struct irqaction layout

We currently use kmalloc-96 slab for struct irqaction allocations on
64bit arches.

This is unfortunate because of possible false sharing and two cache
lines accesses.

Move 'name' and 'dir' fields at the end of the structure, and force a
suitable alignement.

Hot path fields now use one cache line on x86_64.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Reviewed-by: Andi Kleen <andi@firstfloor.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1288865628.2659.69.camel@edumazet-laptop>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>