scsi: gdth: refactor ioc_general

This function is a huge mess with duplicated error handling.  Split out
a few useful helpers and use goto labels to untangle the error handling
and no-data ioctl handling.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>