netlink: remove type-unsafe validation_data pointer

In the netlink policy, we currently have a void *validation_data
that's pointing to different things:
 * a u32 value for bitfield32,
 * the netlink policy for nested/nested array
 * the string for NLA_REJECT

Remove the pointer and place appropriate type-safe items in the
union instead.

While at it, completely dissolve the pointer for the bitfield32
case and just put the value there directly.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>