scsi: lpfc: Avoid -Wstringop-overflow warning

Prevent any potential integer wrapping issue, and avoid a
-Wstringop-overflow warning by using the check_mul_overflow() helper.

drivers/scsi/lpfc/lpfc.h:
837:#define LPFC_RAS_MIN_BUFF_POST_SIZE (256 * 1024)

drivers/scsi/lpfc/lpfc_debugfs.c:
2266 size = LPFC_RAS_MIN_BUFF_POST_SIZE * phba->cfg_ras_fwlog_buffsize;

this can wrap to negative if cfg_ras_fwlog_buffsize is large
enough. And even when in practice this is not possible (due to
phba->cfg_ras_fwlog_buffsize never being larger than 4[1]), the
compiler is legitimately warning us about potentially buggy code.

Fix the following warning seen under GCC-13:
In function ‘lpfc_debugfs_ras_log_data’,
inlined from ‘lpfc_debugfs_ras_log_open’ at drivers/scsi/lpfc/lpfc_debugfs.c:2271:15:
drivers/scsi/lpfc/lpfc_debugfs.c:2210:25: warning: ‘memcpy’ specified bound between 18446744071562067968 and 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overflow=]
2210 | memcpy(buffer + copied, dmabuf->virt,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2211 | size - copied - 1);
| ~~~~~~~~~~~~~~~~~~

Link: https://github.com/KSPP/linux/issues/305
Link: https://lore.kernel.org/linux-hardening/[e-mail]/ [1]
Co-developed-by: Kees Cook <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Link: https://lore.kernel.org/r/ZHkseX6TiFahvxJA@work
Reviewed-by: Justin Tee <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/lpfc/lpfc_debugfs.c

···
 		goto out;
 	}
 	spin_unlock_irq(&phba->hbalock);
-	debug = kmalloc(sizeof(*debug), GFP_KERNEL);
+
+	if (check_mul_overflow(LPFC_RAS_MIN_BUFF_POST_SIZE,
+			       phba->cfg_ras_fwlog_buffsize, &size))
+		goto out;
+
+	debug = kzalloc(sizeof(*debug), GFP_KERNEL);
 	if (!debug)
 		goto out;
 
-	size = LPFC_RAS_MIN_BUFF_POST_SIZE * phba->cfg_ras_fwlog_buffsize;
 	debug->buffer = vmalloc(size);
 	if (!debug->buffer)
 		goto free_debug;