perf header: Allow attributes to be written after data

With a file, to write data an offset needs to be known. Typically data
follows the event attributes in a file.

However, if processing a pipe the number of event attributes may not be
known.

It is convenient in that case to write the attributes after the data.

Expand perf_session__do_write_header() to allow this when the data
offset and size are known.

This approach may be useful for more than just taking a pipe file to
write into a data file, `perf inject --itrace` will reserve and
additional 8kb for attributes, which would be unnecessary if the
attributes were written after the data.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yanteng Si <siyanteng@loongson.cn>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Link: https://lore.kernel.org/r/20240829150154.37929-6-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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