[SCSI] use the scsi data buffer length to extract transfer size

Commit 8846bab1 introduced a helper that can be used to query the
wire transfer size for a SCSI command taking protection information into
account.

However, some commands do not have a 1:1 mapping between the block range
they work on and the payload size (discard, write same). After the
scatterlist has been set up these requests use __data_len to store the
number of bytes to report completion on. This means that callers of
scsi_transfer_length() would get the wrong byte count for these types of
requests.

To overcome this we make scsi_transfer_length() use the scatterlist
length in the scsi_data_buffer as basis for the wire transfer
calculation instead of __data_len.

Reported-by: Christoph Hellwig <hch@infradead.org>
Debugged-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Fixes: d77e6535
Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>