selftests/bpf: Mark tc_redirect selftest as serial

It seems to cause a lot of harm to kprobe/tracepoint selftests. Yucong
mentioned before that it does manipulate sysfs, which might be the
reason. So let's mark it as serial, though ideally it would be less
intrusive on the system at test.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211022223228.99920-4-andrii@kernel.org