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