selftests/bpf: make veristat skip non-BPF and failing-to-open BPF objects

selftests/bpf: make veristat skip non-BPF and failing-to-open BPF objects

Make veristat ignore non-BPF object files. This allows simpler
mass-verification (e.g., `sudo ./veristat *.bpf.o` in selftests/bpf
directory). Note that `sudo ./veristat *.o` would also work, but with
selftests's multiple copies of BPF object files (.bpf.o and
.bpf.linked{1,2,3}.o) it's 4x slower.

Also, given some of BPF object files could be incomplete in the sense
that they are meant to be statically linked into final BPF object file
(like linked_maps, linked_funcs, linked_vars), note such instances in
stderr, but proceed anyways. This seems like a better trade off between
completely silently ignoring BPF object file and aborting
mass-verification altogether.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220923175913.3272430-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>