scsi: megaraid_sas: Avoid a couple -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the new TRAILING_OVERLAP() helper to fix the following warnings:

drivers/scsi/megaraid/megaraid_sas_fusion.h:1153:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/scsi/megaraid/megaraid_sas_fusion.h:1198:32: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

This helper creates a union between a flexible-array member (FAM) and a
set of MEMBERS that would otherwise follow it --in this case 'struct
MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES_DYN]' and 'struct
MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES]' in the corresponding
structures.

This overlays the trailing members onto the FAM (struct MR_LD_SPAN_MAP
ldSpanMap[];) while keeping the FAM and the start of MEMBERS aligned.

The static_assert() ensures this alignment remains, and it's
intentionally placed inmediately after the corresponding structures --no
blank line in between.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://patch.msgid.link/aM1E7Xa8qYdZ598N@kspp
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/megaraid/megaraid_sas_fusion.h

···
 } LD_SPAN_INFO, *PLD_SPAN_INFO;
 
 struct MR_FW_RAID_MAP_ALL {
-	struct MR_FW_RAID_MAP raidMap;
-	struct MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES];
+	/* Must be last --ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct MR_FW_RAID_MAP, raidMap, ldSpanMap,
+		struct MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES];
+	);
 } __attribute__ ((packed));
+static_assert(offsetof(struct MR_FW_RAID_MAP_ALL, raidMap.ldSpanMap) ==
+	      offsetof(struct MR_FW_RAID_MAP_ALL, ldSpanMap));
 
 struct MR_DRV_RAID_MAP {
 	/* total size of this structure, including this field.
···
  * And it is mainly for code re-use purpose.
  */
 struct MR_DRV_RAID_MAP_ALL {
-
-	struct MR_DRV_RAID_MAP raidMap;
-	struct MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES_DYN];
+	/* Must be last --ends in a flexible-array member. */
+	TRAILING_OVERLAP(struct MR_DRV_RAID_MAP, raidMap, ldSpanMap,
+		struct MR_LD_SPAN_MAP ldSpanMap[MAX_LOGICAL_DRIVES_DYN];
+	);
 } __packed;
+static_assert(offsetof(struct MR_DRV_RAID_MAP_ALL, raidMap.ldSpanMap) ==
+	      offsetof(struct MR_DRV_RAID_MAP_ALL, ldSpanMap));