scsi: scsi_debug: Replace kzalloc() + copy_from_user() with memdup_user_nul()

Replace kzalloc() followed by copy_from_user() with memdup_user_nul() to
improve and simplify sdebug_error_write().

No functional changes intended.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>