lib/repo: Do free space math under lock in error path

We were referencing the txn bits outside of the lock in the error
path. Generally shouldn't matter, but e.g. Rust wouldn't let us do this, and
race detector tooling will warn about it.

Closes: #1632
Approved by: jlebon

src/libostree/ostree-repo-commit.c

@@ -896,8 +896,9 @@ write_content_object (OstreeRepo *self,
  const fsblkcnt_t object_blocks = (size / self->txn.blocksize) + 1;
  if (object_blocks > self->txn.max_blocks)
  {
+ guint64 bytes_required = (guint64)object_blocks * self->txn.blocksize;
  g_mutex_unlock (&self->txn_lock);
- g_autofree char *formatted_required = g_format_size ((guint64)object_blocks * self->txn.blocksize);
+ g_autofree char *formatted_required = g_format_size (bytes_required);
  if (self->min_free_space_percent > 0)
  return glnx_throw (error, "min-free-space-percent '%u%%' would be exceeded, %s more required",
  self->min_free_space_percent, formatted_required);
@@ -1609,8 +1610,9 @@ ostree_repo_prepare_transaction (OstreeRepo *self,
  self->txn.max_blocks = bfree - reserved_blocks;
  else
  {
+ guint64 bytes_required = bfree * self->txn.blocksize;
  g_mutex_unlock (&self->txn_lock);
- g_autofree char *formatted_free = g_format_size (bfree * self->txn.blocksize);
+ g_autofree char *formatted_free = g_format_size (bytes_required);
  if (self->min_free_space_percent > 0)
  return glnx_throw (error, "min-free-space-percent '%u%%' would be exceeded, %s available",
  self->min_free_space_percent, formatted_free);