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/fsck_unshare_blocks.cpp b/fsck_unshare_blocks.cpp
index acc453c..ce6940a 100644
--- a/fsck_unshare_blocks.cpp
+++ b/fsck_unshare_blocks.cpp
@@ -121,7 +121,7 @@
 
   // Temporarily mount system so we can copy e2fsck_static.
   std::string system_root = get_system_root();
-  bool mounted = ensure_path_mounted_at(system_root.c_str(), "/mnt/system") != -1;
+  bool mounted = ensure_path_mounted_at(system_root, "/mnt/system") != -1;
   partitions.push_back(system_root);
 
   if (!mounted) {