libbpf: Fix memleak in libbpf_netlink_recv()

Ensure that libbpf_netlink_recv() frees dynamically allocated buffer in
all code paths.

Fixes: 9c3de619 ("libbpf: Use dynamically allocated buffer when receiving netlink messages")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20220217073958.276959-1-andrii@kernel.org