ovl: generalize the lower_layers[] array

ovl: generalize the lower_layers[] array

Rename lower_layers[] array to layers[], extend its size by one and
initialize layers[0] with upper layer values.  Lower layers are now
addressed with index 1..numlower.  layers[0] is reserved even with lower
only overlay.

[SzM: replace ofs->numlower with ofs->numlayer, the latter's value is
incremented by one]

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