selftests/bpf: Allow building bpf tests with CONFIG_XFRM_INTERFACE=[m|n]

It is useful to use vmlinux.h in the xfrm_info test like other kfunc
tests do. In particular, it is common for kfunc bpf prog that requires
to use other core kernel structures in vmlinux.h

Although vmlinux.h is preferred, it needs a ___local flavor of
struct bpf_xfrm_info in order to build the bpf selftests
when CONFIG_XFRM_INTERFACE=[m|n].

Cc: Eyal Birger <eyal.birger@gmail.com>
Fixes: 90a3a05eb33f ("selftests/bpf: add xfrm_info tests")
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20221206193554.1059757-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: add xfrm_info tests

Test the xfrm_info kfunc helpers.

The test setup creates three name spaces - NS0, NS1, NS2.

XFRM tunnels are setup between NS0 and the two other NSs.

The kfunc helpers are used to steer traffic from NS0 to the other
NSs based on a userspace populated bpf global variable and validate
that the return traffic had arrived from the desired NS.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Link: https://lore.kernel.org/r/20221203084659.1837829-5-eyal.birger@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>