libbpf: Fix segfault due to libelf functions not setting errno

Libelf functions do not set errno on failure. Instead, it relies on its
internal _elf_errno value, that can be retrieved via elf_errno (or the
corresponding message via elf_errmsg()). From "man libelf":

If a libelf function encounters an error it will set an internal
error code that can be retrieved with elf_errno. Each thread
maintains its own separate error code. The meaning of each error
code can be determined with elf_errmsg, which returns a string
describing the error.

As a consequence, libbpf should not return -errno when a function from
libelf fails, because an empty value will not be interpreted as an error
and won't prevent the program to stop. This is visible in
bpf_linker__add_file(), for example, where we call a succession of
functions that rely on libelf:

err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
err = err ?: linker_append_sec_data(linker, &obj);
err = err ?: linker_append_elf_syms(linker, &obj);
err = err ?: linker_append_elf_relos(linker, &obj);
err = err ?: linker_append_btf(linker, &obj);
err = err ?: linker_append_btf_ext(linker, &obj);

If the object file that we try to process is not, in fact, a correct
object file, linker_load_obj_file() may fail with errno not being set,
and return 0. In this case we attempt to run linker_append_elf_sysms()
and may segfault.

This can happen (and was discovered) with bpftool:

$ bpftool gen object output.o sample_ret0.bpf.c
libbpf: failed to get ELF header for sample_ret0.bpf.c: invalid `Elf' handle
zsh: segmentation fault (core dumped) bpftool gen object output.o sample_ret0.bpf.c

Fix the issue by returning a non-null error code (-EINVAL) when libelf
functions fail.

Fixes: faf6ed321cf6 ("libbpf: Add BPF static linker APIs")
Signed-off-by: Quentin Monnet <qmo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241205135942.65262-1-qmo@kernel.org