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 <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Nick Terrell <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Yanteng Si <[email protected]>
Cc: Yicong Yang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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