[PATCH] libata: fix READ CAPACITY simulation

* READ CAPACITY (16) implementation fixed.  Result was shifted by two
  bytes.  Carlos Pardo spotted this problem and submitted preliminary
  patch.  Capacity => 2TB is handled correctly now.  (verifid w/ fake
  capacity)

* Use dev->n_sectors instead of re-reading directly from ID data.

* Define and use ATA_SCSI_RBUF_SET() which considers rbuf length.
  This should be done for all simulation functions.  Userland can
  issue any simulated command with arbitrary buffer length.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Carlos Pardo <Carlos.Pardo@siliconimage.com>