UBUNTU: SAUCE: aufs -- Convert to use xattr handlers

UBUNTU: SAUCE: aufs -- Convert to use xattr handlers

Starting with Linux 4.9-rc1 the {get,set,remove}xattr inode
operations have been removed, and filesystems are required to use
xattr handlers instead. There's some partially implemented xattr
handlers commented out in aufs already. Finish those handlers and
convert aufs over to use them.

Fixes FTBFS with 4.9 kernels.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>