selftests/bpf: Add test to verify the fix of kprobe_write_ctx abuse

selftests/bpf: Add test to verify the fix of kprobe_write_ctx abuse

Add a test to verify the issue: kprobe_write_ctx can be abused to modify
struct pt_regs of kernel functions via kprobe_write_ctx=true freplace
progs.

Without the fix, the issue is verified:

kprobe_write_ctx=true freplace prog is allowed to attach to
kprobe_write_ctx=false kprobe prog. Then, the first arg of
bpf_fentry_test1 will be set as 0, and bpf_prog_test_run_opts() gets
-EFAULT instead of 0.

With the fix, the issue is rejected at attach time.

Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260331145353.87606-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>