scsi: uapi: Replace 0-length array with flexible array

Zero-length arrays are deprecated[1]. Replace struct
fc_bsg_host_vendor_reply's "vendor_rsp" 0-length array with a flexible
array. Detected with GCC 13, using -fstrict-flex-arrays=3:

drivers/scsi/qla2xxx/qla_isr.c: In function 'qla25xx_process_bidir_status_iocb.isra':
drivers/scsi/qla2xxx/qla_isr.c:3117:54: warning: array subscript 0 is outside array bounds of '__u32[0]' {aka 'unsigned int[]'} [-Warray-bounds=]
 3117 |         bsg_reply->reply_data.vendor_reply.vendor_rsp[0] = rval;
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
In file included from drivers/scsi/qla2xxx/qla_def.h:34,
                 from drivers/scsi/qla2xxx/qla_isr.c:6:
include/uapi/scsi/scsi_bsg_fc.h:219:15: note: while referencing 'vendor_rsp'
  219 |         __u32 vendor_rsp[0];
      |               ^~~~~~~~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230105233042.never.913-kees@kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

include/uapi/scsi/scsi_bsg_fc.h

@@ -216,7 +216,7 @@ struct fc_bsg_host_vendor {
  */
 struct fc_bsg_host_vendor_reply {
 	/* start of vendor response area */
-	__u32 vendor_rsp[0];
+	__DECLARE_FLEX_ARRAY(__u32, vendor_rsp);
 };