selftests/bpf: Verify ca_name of struct mptcp_sock

This patch verifies another member of struct mptcp_sock, ca_name. Add a
new function get_msk_ca_name() to read the sysctl tcp_congestion_control
and verify it in verify_msk().

v3: Access the sysctl through the filesystem to avoid compatibility
    issues with the busybox sysctl command.

v4: use ASSERT_* instead of CHECK_FAIL (Andrii)

v5: use ASSERT_STRNEQ() instead of strncmp() (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/20220519233016.105670-7-mathew.j.martineau@linux.intel.com