libbpf: Fix various errors and warning reported by checkpatch.pl

Fix a bunch of warnings and errors reported by checkpatch.pl, to make it
easier to spot new problems.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191121070743.1309473-4-andriin@fb.com