fs_mgr: remount: system is root

For devices without overlayfs and system is root (Hikey, Hikey960)
correct "/system" to "/" if that is the mount point.

Test: adb-remount-test.sh
Bug: 129720614
Change-Id: I3bdbd5ea7b70d03477055e60521d8132b486c904
diff --git a/fs_mgr/fs_mgr_remount.cpp b/fs_mgr/fs_mgr_remount.cpp
index 24044d8..093d44d 100644
--- a/fs_mgr/fs_mgr_remount.cpp
+++ b/fs_mgr/fs_mgr_remount.cpp
@@ -340,6 +340,7 @@
                 blk_device = rentry.blk_device;
                 break;
             }
+            // Find overlayfs mount point?
             if ((mount_point == "/") && (rentry.mount_point == "/system")) {
                 blk_device = rentry.blk_device;
                 mount_point = "/system";
@@ -352,6 +353,12 @@
         }
         fs_mgr_set_blk_ro(blk_device, false);
 
+        // Find system-as-root mount point?
+        if ((mount_point == "/system") && !GetEntryForMountPoint(&mounts, mount_point) &&
+            GetEntryForMountPoint(&mounts, "/")) {
+            mount_point = "/";
+        }
+
         // Now remount!
         if (::mount(blk_device.c_str(), mount_point.c_str(), entry.fs_type.c_str(), MS_REMOUNT,
                     nullptr) == 0) {