fsnotify: Check on dump that file handle can be opened

Some filesystems do not provide open-by-handle functionality. For those,
we should abort fsnotifies dumping, not restoring.

The open_mount() changes are about opening mountpoints inside another
mount namespace.

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