eta: calculate aggregate bw statistics even when eta is disabled

eta: calculate aggregate bw statistics even when eta is disabled

The --bandwidth-log command-line option instructs fio to generate
aggregate bandwidth log files. These measurements are recorded by the
code generating the eta status line. When eta is disabled the aggregate
bandwidth log measurements are not calculated. Change the eta code to
record the measurements even when eta is not needed.

eta is disabled under these conditions
- explicitly with --eta=never
- STDOUT is not a TTY (shell redirection, nohup, etc)
- output format excludes normal output

Fixes: https://github.com/axboe/fio/issues/1599
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>