netlink: limit recursion depth in policy validation

Now that we have nested policies, we can theoretically
recurse forever parsing attributes if a (sub-)policy
refers back to a higher level one. This is a situation
that has happened in nl80211, and we've avoided it there
by not linking it.

Add some code to netlink parsing to limit recursion depth,
allowing us to safely change nl80211 to actually link the
nested policy, which in turn allows some code cleanups.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
4 files changed