[SCSI] libsas: fix port->dev_list locking

port->dev_list maintains a list of devices attached to a given sas root port.
It needs to be mutated under a lock as contexts outside of the
single-threaded-libsas-workqueue access the list via sas_find_dev_by_rphy().
Fixup locations where the list was being mutated without a lock.

This is a follow-up to commit 5911e963 "[SCSI] libsas: remove expander
from dev list on error", where Luben noted [1]:

    > 2/ We have unlocked list manipulations in sas_ex_discover_end_dev(),
    > sas_unregister_common_dev(), and sas_ex_discover_end_dev()

    Yes, I can see that and that is very unfortunate.

[1]: http://marc.info/?l=linux-scsi&m=131480962006471&w=2

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>
diff --git a/include/scsi/libsas.h b/include/scsi/libsas.h
index c4b7cd0..6a308d42 100644
--- a/include/scsi/libsas.h
+++ b/include/scsi/libsas.h
@@ -656,7 +656,7 @@
 int  sas_discover_sata(struct domain_device *);
 int  sas_discover_end_dev(struct domain_device *);
 
-void sas_unregister_dev(struct domain_device *);
+void sas_unregister_dev(struct asd_sas_port *port, struct domain_device *);
 
 void sas_init_dev(struct domain_device *);