[$] Parallel directory operations

Allowing directories to be modified in parallel was the topic of Jeff
Layton's filesystem-track session at the 2025 Linux Storage, Filesystem,
Memory Management, and BPF Summit (LSFMM+BPF). There are certain use
cases, including for the NFS and Lustre filesystems, as mentioned in a patch set
referenced in the topic
proposal, where contention in creating multiple files in a directory is
causing noticeable performance problems. In some testing, Layton has found
that the inode read-write semaphore (i_rwsem) for the directory is
serializing operations; he wanted to discuss alternatives.