net/smc: mutex to protect the lgr against parallel reconfigurations

Introduce llc_conf_mutex in the link group which is used to protect the
buffers and lgr states against parallel link reconfiguration.
This ensures that new connections do not start to register buffers with
the links of a link group when link creation or termination is running.

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>