selftests: mptcp: pm: validate new limits

selftests: mptcp: pm: validate new limits

These limits have been recently updated, from 8 to:

- 64 for the subflows and accepted add_addr

- 255 for the MPTCP endpoints

These modifications validate the new limits, but are also compatible
with the previous ones, to be able to continue to validate stable kernel
using the last version of the selftests. That's why new variables are
now used instead of hard-coded values.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260508-net-next-mptcp-pm-inc-limits-v1-7-c84e3fdf9b6a@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>