sysroot: Don't individually fsync dirs in checkout, rely on syncfs

Originally, a lot of the `fsync()` calls here were added for the
wrong reason - I was chasing a bug that ended up being the extlinux
bootloader not parsing 64 bit ext4 filesystems.  But since it looked
like corruption, I tried adding a lot more `fsync()` calls.

All we should have to do is use `syncfs()`.  If that doesn't work,
it's a kernel bug.

I'm making this change because skipping the individual fsyncs can be a
major performance win - it's easier for the FS to optimize, we do more
in parallel, etc.

https://bugzilla.gnome.org/show_bug.cgi?id=757117

src/libostree/ostree-repo.c

@@ -2384,6 +2384,19 @@ ostree_repo_set_disable_fsync (OstreeRepo    *self,
   self->disable_fsync = disable_fsync;
 }
 
+/**
+ * ostree_repo_get_disable_fsync:
+ * @self: An #OstreeRepo
+ *
+ * For more information see ostree_repo_set_disable_fsync().
+ *
+ * Returns: Whether or not fsync() is enabled for this repo.
+ */
+gboolean
+ostree_repo_get_disable_fsync (OstreeRepo    *self)
+{
+  return self->disable_fsync;
+}
 
 /* Replace the contents of a file, honoring the repository's fsync
  * policy.