selftests/bpf: Add err.h header

Moving error macros from profiler.inc.h to new err.h header.
It will be used in following changes.

Also adding PTR_ERR macro that will be used in following changes.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230331093157.1749137-2-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>