netfilter: nf_tables: Add notifications for hook changes

Notify user space if netdev hooks are updated due to netdev add/remove
events. Send minimal notification messages by introducing
NFT_MSG_NEWDEV/DELDEV message types describing a single device only.

Upon NETDEV_CHANGENAME, the callback has no information about the
interface's old name. To provide a clear message to user space, include
the hook's stored interface name in the notification.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>