[SCSI] move ULD attachment into the prep function

One of the intents of the block prep function was to allow ULDs to use
it for preprocessing.  The original SCSI model was to have a single prep
function and add a pointer indirect filter to build the necessary
commands.  This patch reverses that, does away with the init_command
field of the scsi_driver structure and makes ULDs attach directly to the
prep function instead.  The value is really that it allows us to begin
to separate the ULDs from the SCSI mid layer (as long as they don't use
any core functions---which is hard at the moment---a ULD doesn't even
need SCSI to bind).

Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>