libbpf: Rename bpf_prog_attach_xattr() to bpf_prog_attach_opts()

All xattr APIs are being dropped, let's converge to the convention used in
high-level APIs and rename bpf_prog_attach_xattr to bpf_prog_attach_opts.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220107184604.3668544-2-christylee@fb.com