[$] Per-vector software-interrupt masking

Software interrupts (or "softirqs") are one of the oldestdeferred-execution mechanisms in the kernel, and that age shows at times.Some developers have occasionally been heard to mutter about removing them, butsoftirqs are too deeply embedded into how the kernel works to be easily rippedout; most developers just leave them alone. So the recent per-vectorsoftirq masking patch set from Frederic Weisbecker is noteworthy as anexception to that rule. Weisbecker is not getting rid of softirqs, but heis trying to reduce their impact and improve their latency.