The tcf_exts API abstracts extensions such as actions/policers
into a generic layer and reduces the knowledge inside classifiers
to the minimum required. It isolates the validation code into
its own function to allow classifiers to validate all input
data before making changes and thus avoids the need to undo changes
if a extension configuration request cannot be fullfilled.

Adds missing locking when adding a action/police extension to an
already existing filter. Acquiring  dev->queue_lock makes sure we
don't change the action/police in the middle of a classification.
Noted by Patrick McHardy.

As a nice side effect, using this API removes the existing
ifdef clutter.

Usage:
  The classifier holds struct tcf_exts which may be empty if no
  extensions are compiled in. It then calls tcf_exts_validate
  when a new change request was received and provides a temporary
  tcf_exts copy to store the change requests. Given it succeeded
  the classifier may change its own parameters and at the end
  call tcf_exts_change to commit the changes and replace the
  existing extension configuration with the new one. The classifier
  is responsible to destroy his temporary copy if any of its own
  validation checks fail.

  The classifier specific TLV types must be exported to the extensions
  API via tcf_ext_map.

  Destroying the extensions is as easy as calling tcf_exts_destroy.

  The extensions are executed by the classifier by calling tcf_exts_exec
  which must be done as the last thing after making sure the
  filter matches. Note: A classifier might take further actions after
  the execution to tcf_exts_exec such as correcting its own cache to
  avoid caching results which could have been influenced by the extensions.

  tcf_exts_exec returns a negative error code if the filter must be
  considered unmatched, 0 on normal execution or a positive classifier
  return code (TC_ACT_*) which must be returned to the underlying layer
  as-is.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>