UBI: rename sv to av

After re-naming the 'struct ubi_scan_volume' we should adjust all variables
named 'sv' to something else, because 'sv' stands for "scanning volume".
Let's rename it to 'av' which stands for "attaching volume" which is
a bit more consistent and has the same length, which makes re-naming easy.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>