perf db-export: Rename db_export__comm() to db_export__exec_comm()

Rename db_export__comm() to db_export__exec_comm() to better reflect
what it does 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/20190710085810.1650-3-adrian.hunter@intel.com


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