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 <[email protected]>
Signed-off-by: Antony Antony <[email protected]>
Signed-off-by: Daniel Xu <[email protected]>
Link: https://lore.kernel.org/r/884bde1d9a351d126a3923886b945ea6b1b0776b.1702593901.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <[email protected]>

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