Merge branch 'selftests/bpf: make BPF_CFLAGS stricter with -Wall'

Andrii Nakryiko says:

====================

Make BPF-side compiler flags stricter by adding -Wall. Fix tons of small
issues pointed out by compiler immediately after that. That includes newly
added bpf_for(), bpf_for_each(), and bpf_repeat() macros.
====================

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