llc: Fix races between llc2 handler use and (un)registration

When registering the handlers, any state they rely on must be
completely initialised first.  When unregistering, we must wait until
they are definitely no longer running.  llc_rcv() must also avoid
reading the handler pointers again after checking for NULL.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>