scsi: core: Rename and move get_scsi_ml_byte()

SCSI has two different getters:

 - get_XXX_byte() (in scsi_cmnd.h) which takes a struct scsi_cmnd *, and

 - XXX_byte() (in scsi.h) which takes a scmd->result.

The proper name for get_scsi_ml_byte() should thus be without the get_
prefix, as it takes a scmd->result. Rename the function to rectify this.
(This change was suggested by Mike Christie.)

Additionally, move get_scsi_ml_byte() to scsi_priv.h since both scsi_lib.c
and scsi_error.c will need to use this helper in a follow-up patch.

Cc: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Link: https://lore.kernel.org/r/20230511011356.227789-6-nks@flawful.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>