ata: sata_fsl: Use struct_group() for memcpy() region

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() in struct command_desc around members acmd and fill,
so they can be referenced together. This will allow memset(), memcpy(),
and sizeof() to more easily reason about sizes, improve readability,
and avoid future warnings about writing beyond the end of acmd:

In function 'fortify_memset_chk',
    inlined from 'sata_fsl_qc_prep' at drivers/ata/sata_fsl.c:534:3:
./include/linux/fortify-string.h:199:4: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
  199 |    __write_overflow_field();
      |    ^~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NDamien Le Moal <damien.lemoal@opensource.wdc.com>

drivers/ata/sata_fsl.c

@@ -246,8 +246,10 @@ enum {
 struct command_desc {
 	u8 cfis[8 * 4];
 	u8 sfis[8 * 4];
-	u8 acmd[4 * 4];
-	u8 fill[4 * 4];
+	struct_group(cdb,
+		u8 acmd[4 * 4];
+		u8 fill[4 * 4];
+	);
 	u32 prdt[SATA_FSL_MAX_PRD_DIRECT * 4];
 	u32 prdt_indirect[(SATA_FSL_MAX_PRD - SATA_FSL_MAX_PRD_DIRECT) * 4];
 };
@@ -531,8 +533,8 @@ static enum ata_completion_errors sata_fsl_qc_prep(struct ata_queued_cmd *qc)
 	/* setup "ACMD - atapi command" in cmd. desc. if this is ATAPI cmd */
 	if (ata_is_atapi(qc->tf.protocol)) {
 		desc_info |= ATAPI_CMD;
-		memset((void *)&cd->acmd, 0, 32);
-		memcpy((void *)&cd->acmd, qc->cdb, qc->dev->cdb_len);
+		memset(&cd->cdb, 0, sizeof(cd->cdb));
+		memcpy(&cd->cdb, qc->cdb, qc->dev->cdb_len);
 	}
 
 	if (qc->flags & ATA_QCFLAG_DMAMAP)