net/smc: enqueue all received LLC messages

Introduce smc_llc_enqueue() to enqueue LLC messages, and adapt
smc_llc_rx_handler() to enqueue all received LLC messages.
smc_llc_enqueue() also makes it possible to enqueue LLC messages from
local code.

Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>