ovl: fix possible use after free on redirect dir lookup

ovl_lookup_layer() iterates on path elements of d->name.name
but also frees and allocates a new pointer for d->name.name.

For the case of lookup in upper layer, the initial d->name.name
pointer is stable (dentry->d_name), but for lower layers, the
initial d->name.name can be d->redirect, which can be freed during
iteration.

[SzM]
Keep the count of remaining characters in the redirect path and calculate
the current position from that.  This works becuase only the prefix is
modified, the ending always stays the same.

Fixes: 02b69b28

 ("ovl: lookup redirects")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>