Mount /mnt/scracth with -o sync,nodiscard

"-o discard" issues UNMAP commands to loopback resulting in punch_hole
on the pinned file in /data. That will break the pinned block map.

Bug: 210589189
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: Ia927c43fc75164ce5929173f5740737eac4de484

fs_mgr/fs_mgr_overlayfs.cpp

@@ -880,6 +880,8 @@ bool fs_mgr_overlayfs_mount_scratch(const std::string& device_path, const std::s
             errno = save_errno;
         }
         entry.flags &= ~MS_RDONLY;
+        entry.flags |= MS_SYNCHRONOUS;
+        entry.fs_options = "nodiscard";
         fs_mgr_set_blk_ro(device_path, false);
     }
     // check_fs requires apex runtime library