perf pmu: Don't eagerly parse event terms

perf pmu: Don't eagerly parse event terms

When an event/alias is created for a PMU the terms are eagerly parsed
using parse_events_terms. For a command like perf stat or perf record,
the particular event/alias will be found, the terms parsed, the
terms cloned for use in the event parsing, and then the terms used to
configure the perf_event_attr. Events/aliases may be eagerly loaded,
such as from sysfs or in perf list, in which case the aliases terms
will be little or never used. To avoid redundant work, to avoid
cloning, and to reduce memory overhead, hold the terms for an event as
a string until they need handling as a term list. This may introduce
duplicate parsing if an event is repeated in a list, but this
situation is expected to be uncommon.

Measuring the number of instructions before and after with a sysfs
event and perf stat, there is a minor reduction in the number of
instructions executed by 0.3%.

Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>