selftests/bpf: Add pid limit for mptcpify prog

In order to prevent mptcpify prog from affecting the running results
of other BPF tests, a pid limit was added to restrict it from only
modifying its own program.

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/8987e2938e15e8ec390b85b5dcbee704751359dc.1712054986.git.tanggeliang@kylinos.cn