bpf: selftests: test_tunnel: Use vmlinux.h declarations

vmlinux.h declarations are more ergnomic, especially when working with
kfuncs. The uapi headers are often incomplete for kfunc definitions.

This commit also switches bitfield accesses to use CO-RE helpers.
Switching to vmlinux.h definitions makes the verifier very
unhappy with raw bitfield accesses. The error is:

    ; md.u.md2.dir = direction;
    33: (69) r1 = *(u16 *)(r2 +11)
    misaligned stack access off (0x0; 0x0)+-64+11 size 2

Fix by using CO-RE-aware bitfield reads and writes.

Co-developed-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/884bde1d9a351d126a3923886b945ea6b1b0776b.1702593901.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>