selftests/bpf: Cover tracing implicit kfunc args

selftests/bpf: Cover tracing implicit kfunc args

KF_IMPLICIT_ARGS kfuncs have a BPF-call prototype and a real kernel
target prototype. Add a tracing selftest that attaches fentry and fexit
programs to bpf_kfunc_implicit_arg(), runs a syscall BPF program that
calls it, and checks that the tracing context exposes both the explicit
argument and the implicit prog aux pointer.

Co-developed-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://patch.msgid.link/20260713235223.1639022-3-ihor.solodrai@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>