net/smc: llc_add_link_work to handle ADD_LINK LLC requests

Introduce a work that is scheduled when a new ADD_LINK LLC request is
received. The work will call either the SMC client or SMC server
ADD_LINK processing.

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>