[Driver] Flush file in locked area

When writing report file by option -proc-stat-report some part of output
can be written to unlocked file because destructor of raw_fd_ostream
calls `flush()`. In high thread contention environment it can result in
file operation failure. With this change `flush` is called explicitly when
file is locked, so call of `flush()` in the destructor does not cause
write to file.