perf session: Correct buffer copying when peeking events

When peeking an event, it has a short path and a long path.  The short
path uses the session pointer "one_mmap_addr" to directly fetch the
event; and the long path needs to read out the event header and the
following event data from file and fill into the buffer pointer passed
through the argument "buf".

The issue is in the long path that it copies the event header and event
data into the same destination address which pointer "buf", this means
the event header is overwritten.  We are just lucky to run into the
short path in most cases, so we don't hit the issue in the long path.

This patch adds the offset "hdr_sz" to the pointer "buf" when copying
the event data, so that it can reserve the event header which can be
used properly by its caller.

Fixes: 5a52f33a

 ("perf session: Add perf_session__peek_event()")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210605052957.1070720-1-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>