selftests/net: fix section name when using xdp_dummy.o

selftests/net: fix section name when using xdp_dummy.o

Since commit 1848e3c8193b ("selftests/bpf: Normalize XDP section names in
selftests") the xdp_dummy.o's section name has changed to xdp. But some
tests are still using "section xdp_dummy", which make the tests failed.
Fix them by updating to the new section name.

Fixes: 1848e3c8193b ("selftests/bpf: Normalize XDP section names in selftests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220630062228.3453016-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>