selftests/bpf: Verify token of struct mptcp_sock

This patch verifies the struct member token of struct mptcp_sock. Add a
new member token in struct mptcp_storage to store the token value of the
msk socket got by bpf_skc_to_mptcp_sock(). Trace the kernel function
mptcp_pm_new_connection() by using bpf fentry prog to obtain the msk token
and save it in a global bpf variable. Pass the variable to verify_msk() to
verify it with the token saved in socket_storage_map.

v4:
 - use ASSERT_* instead of CHECK_FAIL (Andrii)
 - skip the test if 'ip mptcp monitor' is not supported (Mat)

v5:
 - Drop 'ip mptcp monitor', trace mptcp_pm_new_connection instead (Martin)
 - Use ASSERT_EQ (Andrii)

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/bpf/20220519233016.105670-6-mathew.j.martineau@linux.intel.com