selftests/net: fix missing xdp_dummy

After commit afef88e6 ("selftests/bpf: Store BPF object files with
.bpf.o extension"), we should use xdp_dummy.bpf.o instade of xdp_dummy.o.

In addition, use the BPF_FILE variable to save the BPF object file name,
which can be better identified and modified.

Fixes: afef88e6 ("selftests/bpf: Store BPF object files with .bpf.o extension")
Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Cc: Daniel Müller <deso@posteo.net>
Signed-off-by: David S. Miller <davem@davemloft.net>