IB/srpt: Fix login-related race conditions

Make sure that sport->mutex is not released between the duplicate
channel check, adding a channel to the channel list and performing
the sport enabled check. Avoid that srpt_disconnect_ch() can be
invoked concurrently with the ib_send_cm_rep() call by
srpt_cm_req_recv().

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>