fsnotify: Helper for opening files by dev:handle pair

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fsnotify: Check on dump that file handle can be opened

Some filesystems do not provide open-by-handle functionality. For those,
we should abort fsnotifies dumping, not restoring.

The open_mount() changes are about opening mountpoints inside another
mount namespace.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
irmap: Reverse dev:inode to path mapping

For inotify/fanotifies we cannot always open inodes by a
handle and have to scan directories searching for the inode
path :(

Fortunately, in most of the containers' cases fsnotifies are
put in "typical" places. These are used as hints for scanner.

The best way to go is use openvz's ploop over such filesystems.
Long term solution is to fix NFS to provide opening by handle.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
fsnotify: Restore name-hinted marks

If we failed to open inode by handle, try doing the irmap
search. If that's successful, dump the "real" path for the
inode.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>