ZTS: random_readwrite test doesn't run correctly

This test uses fio's bssplit mechanism to choose io sizes for the test,
leaving the PERF_IOSIZES variable empty. Because that variable is
empty, the innermost loop in do_fio_run_impl is never executed, and as
a result, this test does the setup but collects no data. Setting the
variable to "bssplit" allows performance data to be gathered.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: John Wren Kennedy <john.kennedy@delphix.com>
Closes #14163