libbpf: Fix lookup_and_delete_elem_flags error reporting

Fix bpf_map_lookup_and_delete_elem_flags() to pass the return code through
libbpf_err_errno() as we do similarly in bpf_map_lookup_and_delete_elem().

Fixes: f12b6543 ("libbpf: Streamline error reporting for low-level APIs")
Signed-off-by: Mehrdad Arshad Rad <arshad.rad@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211104171354.11072-1-arshad.rad@gmail.com