scsi: megaraid_sas: fix panic on loading firmware crashdump

While loading fw crashdump in function fw_crash_buffer_show(), left bytes
in one dma chunk was not checked, if copying size over it, overflow access
will cause kernel panic.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/megaraid/megaraid_sas_base.c

@@ -3163,6 +3163,7 @@ fw_crash_buffer_show(struct device *cdev,
 		(struct megasas_instance *) shost->hostdata;
 	u32 size;
 	unsigned long dmachunk = CRASH_DMA_BUF_SIZE;
+	unsigned long chunk_left_bytes;
 	unsigned long src_addr;
 	unsigned long flags;
 	u32 buff_offset;
@@ -3186,6 +3187,8 @@ fw_crash_buffer_show(struct device *cdev,
 	}
 
 	size = (instance->fw_crash_buffer_size * dmachunk) - buff_offset;
+	chunk_left_bytes = dmachunk - (buff_offset % dmachunk);
+	size = (size > chunk_left_bytes) ? chunk_left_bytes : size;
 	size = (size >= PAGE_SIZE) ? (PAGE_SIZE - 1) : size;
 
 	src_addr = (unsigned long)instance->crash_buf[buff_offset / dmachunk] +