selftests/bpf: Stress test attaching a BPF prog to another BPF prog

selftests/bpf: Stress test attaching a BPF prog to another BPF prog

Add a test that invokes a BPF prog in a loop, while concurrently
attaching and detaching another BPF prog to and from it. This helps
identifying race conditions in bpf_arch_text_poke().

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250716194524.48109-3-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>