roots.cpp: convert to C++ Fstab

Convert code to use C++ Fstab struct and C++ std::strings.

Bug: 62292478
Bug: 118634720
Test: boots
Change-Id: Ibdc1df5831bc885d7c1574419f41af026e49a137
diff --git a/install.cpp b/install.cpp
index 680937d..ce3d4f6 100644
--- a/install.cpp
+++ b/install.cpp
@@ -595,9 +595,9 @@
 
   if (needs_mount) {
     if (path[0] == '@') {
-      ensure_path_mounted(path.substr(1).c_str());
+      ensure_path_mounted(path.substr(1));
     } else {
-      ensure_path_mounted(path.c_str());
+      ensure_path_mounted(path);
     }
   }