fsnotify: open_handle -- Handle multiple mounts with same s_dev

When inotify is laying on uovermounted fs we should walk over
all mountpoints with same s_dev to find openable path.

Note on restore the path is usually already allocated during
dump stage so get_mark_path won't call for open_handle(), in
turn on dump stage the positive return from open_handle()
will cause fsnotify engine to find openable path, thus there
is kind of double work to be optimized in future.

For example we got a container where systemd-udevd inside
opens inotify for /dev/X entry then overmount ./dev path
with slave option and in result irmap engine on predump
can't figure out where the inotify is sitting causing
migrtion to abort.

Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>