Decouple the creation and destruction of the net_device from the order
of discovery and removal of nodes with RFC 2734 unit directories since
there is no reliable order.  The net_device is now created when the
first RFC 2734 unit on a card is discovered, and destroyed when the last
RFC 2734 unit on a card went away.  This includes all remote units as
well as the local unit, which is therefore tracked as a peer now too.

Also, locking around the list of peers is slightly extended to guard
against peer removal.  As a side effect, fwnet_peer.pdg_lock has become
superfluous and is deleted.

Peer data (max_rec, speed, node ID, generation) are updated more
carefully.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>