Merge branch 'libbpf ELF sanity checking improvements'

Andrii Nakryiko says:

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

Few patches fixing various issues discovered by oss-fuzz project fuzzing
bpf_object__open() call. Fixes are mostly focused around additional simple
sanity checks of ELF format: symbols, relos, etc.

v1->v2:
  - address Yonghong's feedback.
====================

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