mount: Fix restore deleted bindmounts for regfiles

 - make sure it doesn't exist using O_EXCL
 - don't forget to close it

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>