cifs: fix possible uninitialized access and race on iface_list

iface[0] was accessed regardless of the count value and without
locking.

* check count before accessing any ifaces
* make copy of iface list (it's a simple POD array) and use it without
  locking.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>