selftests: mptcp: use max_time instead of time

selftests: mptcp: use max_time instead of time

'time' is the local variable of run_test() function, while 'max_time' is
the local variable of do_transfer() function. So in do_transfer(),
$max_time should be used, not $time.

Please note that here $time == $max_time so the behaviour is not changed
but the right variable is used.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>