fsnotify: Use mnt_id when restoring mount mark

When there is a bind mount present on same mountpoint
where mark is laying the device of both mountpoints
is the same so we might end up getting wrong mountpoint.
Instead we should used mount id which is unique among
all mounpoints.

Note it's a fast fix, I need to review fsnotify code
more and make sure all corner cases are covered.

https://bugzilla.openvz.org/show_bug.cgi?id=2974



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