UBI: add slab cache for ubi_scan_leb objects

During scanning UBI allocates one struct ubi_scan_leb object for each PEB,
so it can end up allocating thousands of them. Use slab cache to reduce
memory consumption for these 48-byte objects, because currently used
'kmalloc()' ends up allocating 64 bytes per object, instead of 48.

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