[SCSI] Improve message printing code

Fix a bug where we would consume one byte too many in the message
printing code.
Add support for 256-byte long messages.
Add support for the Modify Bidirectional Data Pointer message.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>