adding percentage format string for disk info module

explicit cast to double, changed format string to percentage

src/print_disk_info.c

@@ -86,6 +86,11 @@ void print_disk_info(yajl_gen json_gen, char *buffer, const char *path, const ch
                         outwalk += print_bytes_human(outwalk, (uint64_t)buf.f_bsize * (uint64_t)buf.f_bavail);
                         walk += strlen("avail");
                 }
+                
+                if (BEGINS_WITH(walk+1, "percentage")) {
+                        outwalk += sprintf(outwalk, "%.01f%%", 100.0 - 100.0 * (double)buf.f_bavail / (double)buf.f_blocks);
+                        walk += strlen("percentage");
+                }
         }
 
         *outwalk = '\0';