libbpf: Fix return zero when elf_begin failed

The error number of elf_begin is omitted when encapsulating the
btf_find_elf_sections function.

Fixes: c86f180ffc99 ("libbpf: Make btf_parse_elf process .BTF.base transparently")
Signed-off-by: Pu Lehui <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/btf.c

···
 
 	elf = elf_begin(fd, ELF_C_READ, NULL);
 	if (!elf) {
+		err = -LIBBPF_ERRNO__FORMAT;
 		pr_warn("failed to open %s as ELF file\n", path);
 		goto done;
 	}