selftests/net: mv bpf/nat6to4.c to net folder

There are some issues with the bpf/nat6to4.c building.

1. It use TEST_CUSTOM_PROGS, which will add the nat6to4.o to
kselftest-list file and run by common run_tests.
2. When building the test via `make -C tools/testing/selftests/
TARGETS="net"`, the nat6to4.o will be build in selftests/net/bpf/
folder. But in test udpgro_frglist.sh it refers to ../bpf/nat6to4.o.
The correct path should be ./bpf/nat6to4.o.
3. If building the test via `make -C tools/testing/selftests/ TARGETS="net"
install`. The nat6to4.o will be installed to kselftest_install/net/
folder. Then the udpgro_frglist.sh should refer to ./nat6to4.o.

To fix the confusing test path, let's just move the nat6to4.c to net folder
and build it as TEST_GEN_FILES.

Fixes: edae34a3ed92 ("selftests net: add UDP GRO fraglist + bpf self-tests")
Tested-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20230118020927.3971864-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>