selftests/bpf: Change bpf_prog_attach_xattr() to bpf_prog_attach_opts()

bpf_prog_attach_opts() is being deprecated and renamed to
bpf_prog_attach_xattr(). Change all selftests/bpf's uage to the new name.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220107184604.3668544-3-christylee@fb.com