selftests/bpf: prevent unused variable warning in bpf_for()

Add __attribute__((unused)) to inner __p variable inside bpf_for(),
bpf_for_each(), and bpf_repeat() macros to avoid compiler warnings about
unused variable.

Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230309054015.4068562-2-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>