selftests/bpf: do not hardcode target rate in test_tc_edt BPF program

selftests/bpf: do not hardcode target rate in test_tc_edt BPF program

test_tc_edt currently defines the target rate in both the userspace and
BPF parts. This value could be defined once in the userspace part if we
make it able to configure the BPF program before starting the test.

Add a target_rate variable in the BPF part, and make the userspace part
set it to the desired rate before attaching the shaping program.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20251128-tc_edt-v2-4-26db48373e73@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>