net/smc: hold conns_lock before calling smc_lgr_register_conn()

After smc_lgr_create(), the newly created link group is added
to smc_lgr_list, thus is accessible from other context.
Although link group creation is serialized by
smc_create_lgr_pending, the new link group may still be accessed
concurrently. For example, if ib_device is no longer active,
smc_ib_port_event_work() will call smc_port_terminate(), which
in turn will call __smc_lgr_terminate() on every link group of
this device. So conns_lock is required here.

Signed-off-by: Huaping Zhou <zhp@smail.nju.edu.cn>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>