fix file mmap (#8865)

Author: xiao-mang

components/dfs/dfs_v2/src/dfs_file_mmap.c

@@ -184,7 +184,7 @@ void page_read(struct rt_varea *varea, struct rt_aspace_io_msg *msg)
                varea->start, varea->size, varea->offset, varea->attr, varea->flag);
 
         ret = dfs_aspace_mmap_read(file, varea, msg);
-        if (ret > 0)
+        if (ret >= 0)
         {
             msg->response.status = MM_FAULT_STATUS_OK;
             if (ret < ARCH_PAGE_SIZE)

components/dfs/dfs_v2/src/dfs_pcache.c

@@ -1105,7 +1105,7 @@ int dfs_aspace_read(struct dfs_file *file, void *buf, size_t count, off_t *pos)
                 }
 
                 len = count > len ? len : count;
-                if (len)
+                if (len > 0)
                 {
                     rt_memcpy(ptr, page->page + *pos - page->fpos, len);
                     ptr += len;