UBI: rename seb to aeb

After re-naming the 'struct ubi_scan_leb' we should adjust all variables
named 'seb' to something else, because 'seb' stands for "scanning eraseblock".
Let's rename it to 'aeb' which stands for "attaching eraseblock" which is
a bit more consistend and has the same length.

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