selftests: mptcp: simult flows: define missing vars

selftests: mptcp: simult flows: define missing vars

The variables 'large', 'small', 'sout', 'cout', 'capout' and 'size' are
used in multiple functions, so they should be clearly defined as global
variables at the top of the file.

This patch redefines them at the beginning of simult_flows.sh.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-6-b6c8a10396bd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>