perf sample: Make sure perf_sample__init/exit are used

perf sample: Make sure perf_sample__init/exit are used

The deferred stack trace code wasn't using perf_sample__init/exit. Add
the deferred stack trace clean up to perf_sample__exit which requires
proper NULL initialization in perf_sample__init. Make the
perf_sample__exit robust to being called more than once by using
zfree. Make the error paths in evsel__parse_sample exit the
sample. Add a merged_callchain boolean to capture that callchain is
allocated, deferred_callchain doen't suffice for this. Pack the struct
variables to avoid padding bytes for this.

Similiarly powerpc_vpadtl_sample wasn't using perf_sample__init/exit,
use it for consistency and potential issues with uninitialized
variables.

Similarly guest_session__inject_events in builtin-inject wasn't using
perf_sample_init/exit. The lifetime management for fetched events is
somewhat complex there, but when an event is fetched the sample should
be initialized and needs exiting on error. The sample may be left in
place so that future injects have access to it.

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>