When migrating container with copying its FS, the inode numbers
and thus their handles wil change. This will make the restore of
inotify/fanotify fail, since they do it via fhandles.

We've already faced the problems with fsnotifies on NFS -- they
don't work there. To address this an irmap cache is created on
pre-dump, so to resolve the issue with changed inodes during
migration, we can force the irmap cache build.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>