[$] Supporting NFS v4.2 WRITE_SAME

At the 2025 Linux Storage, Filesystem, Memory Management, and BPF Summit
(LSFMM+BPF), Anna Schumaker led a discussion about implementing the NFS
v4.2 WRITE_SAME
command in both the NFS client and server. WRITE_SAME is
meant to write large amounts of identical data (e.g. zeroes) to the server
without actually needing to transfer all of it over the wire. In her topic
proposal, Schumaker wondered whether other filesystems needed the
functionality, so that it should be implemented at the virtual filesystem
(VFS) layer, or whether it should simply be handled as an NFS-specific ioctl().