jffs2: Reduce excessive scan of empty blocks

Scanning 1024 bytes to see if an EB is empty is a bit much.
Lower it to 256 bytes and make sure the while loop is
optimized.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>