btrfs: incremental send, move allocation until it's needed in orphan_dir_info

Move the allocation after the search when it's clear that the new entry
will be added.

Signed-off-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>