net/smc: check for pending termination

Avoid to run the processing in smc_lgr_terminate() more than once,
remember when the link group termination is triggered.

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