Add a check to the xdp_link selftest that the kernel rejects replacing an
XDP program with a different program type on link update.

v2:
- Split this out into its own patch.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20220107221115.326171-3-toke@redhat.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>