perf_disable()

[Peter Zijlstra <peterz@infradead.org>]

Hi,

I've been going over perf_disable() usage in kernel/perf_event.c and
wondered if we actually need it at all.

Currently the only thing we seem to require it for is around pmu::enable
calls (and for that powerpc at least does it itself, on x86 we rely on
it to call ->enable_all and reprogram the pmu state).

But I can't really find any NMI races wrt data structures or the like as
seems implied by some comments. 

There is a fun little recursion issue with perf_adjust_period(), where
if we fully removed perf_disable() we could end up calling pmu::stop()
twice and such.

But aside from that it looks to me its mostly about optimizing hardware
writes.

If nobody else known about/can find anything, I'm going to mostly remove
perf_disable() for now and later think about how to optimize the
hardware writes again.