[LLC]: allow applications to get copy of kernel datagrams

It is legal for an application to bind to a SAP that is also being
used by the kernel. This happens if the bridge module binds to the
STP SAP, and the user wants to have a daemon for STP as well.
It is possible to have kernel doing STP on one bridge, but
let application do RSTP on another bridge.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>