netlink: add validation function to policy

Add the ability to have an arbitrary validation function attached
to a netlink policy that doesn't already use the validation_data
pointer in another way.

This can be useful to validate for example the content of a binary
attribute, like in nl80211 the "(information) elements", which must
be valid streams of "u8 type, u8 length, u8 value[length]".

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