[$] Parallel directory operations

Allowing directories to be modified in parallel was the topic of JeffLayton's filesystem-track session at the 2025 Linux Storage, Filesystem,Memory Management, and BPF Summit (LSFMM+BPF). There are certain usecases, including for the NFS and Lustre filesystems, as mentioned in a patch setreferenced in the topicproposal, where contention in creating multiple files in a directory iscausing noticeable performance problems. In some testing, Layton has foundthat the inode read-write semaphore (i_rwsem) for the directory isserializing operations; he wanted to discuss alternatives.