btrfs: split btrfs_insert_delayed_dir_index() into prealloc and commit phases

btrfs: split btrfs_insert_delayed_dir_index() into prealloc and commit phases

Split btrfs_insert_delayed_dir_index() into three functions using a new
btrfs_dir_index_prealloc struct to bundle the pre-allocated resources:

- btrfs_prealloc_delayed_dir_index(): allocates the struct and performs
  the two GFP_NOFS allocations (delayed node + delayed item) that can
  fail with -ENOMEM. Returns the struct, or ERR_PTR on failure.
- btrfs_insert_delayed_dir_index_prealloc(): populates the item data,
  inserts into the rb-tree, and reserves metadata space. Cannot fail
  with -ENOMEM since all allocations were done in the prealloc step.
- btrfs_free_delayed_dir_index_prealloc(): frees pre-allocated
  resources when the caller's btree insertion fails. Tolerates NULL.

The prealloc is returned as a pointer rather than filled into a
caller-provided struct, so that a plain NULL means "no prealloc" and
callers do not need a separate flag to track whether one exists. It is
consumed (and freed) by either the commit or the free helper, so
ownership is unambiguous.

The original btrfs_insert_delayed_dir_index() is refactored into a thin
wrapper that calls the prealloc and commit functions.

This split allows callers to move the fallible memory allocations before
the point of no return (the DIR_ITEM btree insertion), so that -ENOMEM
can be returned cleanly without aborting the transaction.

Assisted-by: LLM
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>