Export the policy used for attribute validation when it fails,
so e.g. for an out-of-range attribute userspace immediately gets
the valid ranges back.
v2 incorporates the suggestion from Jakub to have a function to
estimate the size (netlink_policy_dump_attr_size_estimate()) and
check that it does the right thing on the *normal* policy dumps,
not (just) when calling it from the error scenario.
Tested using nl80211/iw in a few scenarios, seems to work fine
and return the policy back, e.g.
kernel reports: integer out of range
policy: 04 00 0b 00 0c 00 04 00 01 00 00 00 00 00 00 00
^ padding
^ minimum allowed value
policy: 04 00 0b 00 0c 00 05 00 ff ff ff ff 00 00 00 00
^ padding
^ maximum allowed value
policy: 08 00 01 00 04 00 00 00
^ type 4 == U32
for an out-of-range case.
johannes
