fsnotify: Always provide the path for inotify watchees

In debian-8 container we faced the problem -- systemd creates nested
mount namespaces and inotify watchee are resolved into a path which
is inaccessbile on restore, the same happens when pathes where
watchees are living are bind-overmounted. Thus when we try to
restore such watchees we can't open the paths.

Lets do a trick here (thanks a huge to Andrew Vagin for idea and
overall help) -- walk over all mount points which device match
the handle's device and open handle first and test if the path
provided is openable as well for tmpfs and devtmps which do not
save inodes between remounts. After all the inotify objects are
bound to inode so it's irrelevean via which path it's assigned.

https://jira.sw.ru/browse/PSBM-39957



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