net/smc: prevent new connections on link group

When the processing of a DELETE LINK message has started,
new connections should not be added to the link group that
is about to terminate.

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