aufs: mvdown, don't let unprivileged users provoke a WARNING

Move the WARN_ONCE() about mvdown after the capability check.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
aufs: refine XINO truncation

Now the truncation is triggered by the free space of the filesystem
where the XINO file exists.
With introducing a short lived kstatfs cache which is to stop calling
vfs_statfs() so frequently.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Merge remote-tracking branch 'aufs3-standalone/aufs3.2/00base' into aufs3.2/10static
Merge branch 'aufs3.2/10static' into aufs3.2/11proc_map
Merge branch 'aufs3.2/10static' into aufs3.3/10static
Merge branch 'aufs3.2/11proc_map' into aufs3.3/11proc_map
Merge branch 'aufs3.3/10static' into aufs3.3/11proc_map
Merge branch 'aufs3.3/10static' into aufs3.4/10static
Merge branch 'aufs3.3/11proc_map' into aufs3.4/11proc_map
Merge branch 'aufs3.4/10static' into aufs3.4/11proc_map
Merge branch 'aufs3.4/10static' into aufs3.5/10static
Merge branch 'aufs3.4/11proc_map' into aufs3.5/11proc_map
Merge branch 'aufs3.5/10static' into aufs3.5/11proc_map
Merge branch 'aufs3.5/10static' into aufs3.6/10static
Merge branch 'aufs3.5/11proc_map' into aufs3.6/11proc_map
Merge branch 'aufs3.6/10static' into aufs3.6/11proc_map
Merge branch 'aufs3.2/00base' into aufs3.3/00base
Merge branch 'aufs3.3/00base' into aufs3.4/00base
Merge branch 'aufs3.4/00base' into aufs3.5/00base
Merge branch 'aufs3.5/00base' into aufs3.6/00base