netlink: make validation_data const

The validation data is only used within the policy that
should usually already be const, and isn't changed in any
code that uses it. Therefore, make the validation_data
pointer const.

While at it, remove the duplicate variable in the bitfield
validation that I'd otherwise have to change to const.

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