fsnotify: Lookup for watchee path before doing irmap

In case of migrating filesystem with nonpersistent inodes\devices
we may hit the case where we successfully dump the container but
on restore we will fail because block device get another id
(while its contents might be fully sinc'ed). In this case we
will try to lookup device from its number carried in image
and fail.

Instead lets do an optimistic approach -- always try to
fetch the watchee path first and if it fails go down
to old scheme where irmap and native handle steps
into account.

https://jira.sw.ru/browse/PSBM-40871



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