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

logging.cpp

@@ -46,7 +46,7 @@ static const std::string LAST_LOG_FILTER = "recovery/last_log";
 // fopen(3)'s the given file, by mounting volumes and making parent dirs as necessary. Returns the
 // file pointer, or nullptr on error.
 static FILE* fopen_path(const std::string& path, const char* mode) {
-  if (ensure_path_mounted(path.c_str()) != 0) {
+  if (ensure_path_mounted(path) != 0) {
     LOG(ERROR) << "Can't mount " << path;
     return nullptr;
   }