perf db-export: Move export__comm_thread into db_export__sample()

Move call to db_export__comm_thread() from db_export__thread() into
db_export__sample() because it makes the code easier to understand, and
add explanatory comments.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190710085810.1650-7-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>