ovl: index dir act as work dir

With index=on, let index dir act as the work dir for copy up and cleanups.
This will help implementing whiteout inode sharing.

We still create the "work" dir on mount regardless of index=on and it is
used to test the features supported by upper fs.  One reason is that before
the feature tests, we do not know if index could be enabled or not.

The reason we do not use "index" directory also as workdir with index=off
is because the existence of the "index" directory acts as a simple
persistent signal that index was enabled on this filesystem and tools may
want to use that signal.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>