x86/pmu: Introduce __start_event() to drop all of the manual zeroing

Most invocation of start_event() and measure() first sets evt.count=0.
Instead of forcing each caller to ensure count is zeroed, optimize the
count to zero during start_event(), then drop all of the manual zeroing.

Accumulating counts can be handled by reading the current count before
start_event(), and doing something like stuffing a high count to test an
edge case could be handled by an inner helper, __start_event().

For overflow, just open code measure() for that one-off case. Requiring
callers to zero out a field in most common cases isn't exactly flexible.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Like Xu <likexu@tencent.com>
[sean: tag __measure() noinline so its count is stable]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221102225110.3023543-7-seanjc@google.com