selftests/bpf: Test pointer bpf_loop iteration count rejection

Add a verifier test that leaves the raw tracepoint context pointer in R1
when calling bpf_loop(). This is the smallest trigger for the incorrect
precision backtracking: it reuses an existing callback and needs no maps or
userspace setup.

Expect an ordinary scalar-type rejection. Without the verifier fix, the
test instead reaches precision backtracking and reports an internal
"backtracking misuse" error.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260905014735.1452988-3-memxor@gmail.com
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>