selftests: mptcp: pm nl: remove hardcoded default limits

Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

One of them is the checks of the default limits returned by the MPTCP
in-kernel path-manager. The default values have been modified by commit
72bcbc46 ("mptcp: increase default max additional subflows to 2").
Instead of comparing with hardcoded values, we can get the default one
and compare with them.

Note that if we expect to have the latest version, we continue to check
the hardcoded values to avoid unexpected behaviour changes.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: eedbc685

 ("selftests: add PM netlink functional tests")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>