[SCSI] gdth: Setup proper per-command private data

  - scsi_cmnd and specifically ->SCp of, where heavily abused
    with internal meaning members and flags. So introduce a new
    struct gdth_cmndinfo, put it on ->host_scribble and define a
    gdth_cmnd_priv() accessor to retrieve it from a scsi_cmnd.

  - The structure now holds two members:
    internal_command - replaces the IS_GDTH_INTERNAL_CMD() croft.
    sense_paddr - which was a 64-bit spanning on 2 32-bit members of SCp.
    More overloaded members from SCp and scsi_cmnd will be moved in a later
    patch (For easy review).

  - Split up gdth_queuecommand to an additional internal_function. The later
    is the one called by gdth_execute(). This will be more evident later in
    the scsi accessors patch, but it also facilitates in the differentiation
    between internal_command and external. And the setup of gdth_cmndinfo of
    each command.

Signed-off-by Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>