smb3: interface count displayed incorrectly

The "Server interfaces" count in /proc/fs/cifs/DebugData increases
as the interfaces are requeried, rather than being reset to the new
value.  This could cause a problem if the server disabled
multichannel as the iface_count is checked in try_adding_channels
to see if multichannel still supported.

Also fixes a coverity warning:

Addresses-Coverity: 1526374 ("Concurrent data access violations  (MISSING_LOCK)")
Cc: <stable@vger.kernel.org>
Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -530,6 +530,7 @@ parse_server_interfaces(struct network_interface_info_ioctl_rsp *buf,
 	p = buf;
 
 	spin_lock(&ses->iface_lock);
+	ses->iface_count = 0;
 	/*
 	 * Go through iface_list and do kref_put to remove
 	 * any unused ifaces. ifaces in use will be removed
@@ -651,9 +652,9 @@ parse_server_interfaces(struct network_interface_info_ioctl_rsp *buf,
 			kref_put(&iface->refcount, release_iface);
 		} else
 			list_add_tail(&info->iface_head, &ses->iface_list);
-		spin_unlock(&ses->iface_lock);
 
 		ses->iface_count++;
+		spin_unlock(&ses->iface_lock);
 		ses->iface_last_update = jiffies;
 next_iface:
 		nb_iface++;