stat: remove unnecessary bool parameter to sum_thread_stats()

We can deduce if it is the first struct io_stat src being added to the
struct io_stat dst by checking if the current amount of samples in dst
is zero.

Therefore, remove the bool parameter "first" to sum_stat().
Since sum_stat() was the only user of the bool parameter "first" to
the sum_thread_stats() function, we can remove it from sum_thread_stats()
as well.

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220110090133.69955-1-Niklas.Cassel@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>