bpf_perf_object__next() folded the last element in the list test with the
empty list test. However, this meant that offsets were computed against
null and that a struct list_head was compared against a 'struct
bpf_perf_object'.

Working around this with clang's undefined behavior sanitizer required
-fno-sanitize=null and -fno-sanitize=object-size.

Remove the undefined behavior by using the regular Linux list APIs and
handling the starting case separately from the end testing case.

Looking at uses like bpf_perf_object__for_each(), as the constant NULL
or non-NULL argument can be constant propagated, the code is no less
efficient.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Christy Lee <christylee@fb.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Miaoqian Lin <linmq006@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Rix <trix@redhat.com>
Cc: bpf@vger.kernel.org
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20220726220921.2567761-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>