[SCSI] fix oops in scsi_release_buffers()

I found one other thing that needs to be fixed.  The call to
scsi_release_buffers in scsi_unprep_request causes an oops, because the
sgtable has already been freed in scsi_io_completion.  The following patch
is needed.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>