aufs: tiny, convert' debug' into atomic_t

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 'aufs3.2/11proc_map' into aufs3.2/18public