selftests: ublk: fix fio arguments in run_io_and_recover()

selftests: ublk: fix fio arguments in run_io_and_recover()

run_io_and_recover() invokes fio with --size="${size}", but the variable
size doesn't exist. Thus, the argument expands to --size=, which causes
fio to exit immediately with an error without issuing any I/O. Pass the
value for size as the first argument to the function.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>