The kernel NFS server creates regular files with MKNOD (it
creates-then-opens, so vfs_create routes through fuse_create -> FUSE_MKNOD
when there is no open intent), but mount2 only implemented Create
(FUSE_CREATE, used by local and SMB clients). Without Mknod every NFS file
creation failed with ENOTSUPP. This mirrors the cmd/mount mknod handler from
#2115.
Before: touch through an NFS-exported mount2 mount failed with ENOTSUPP.
After: files create normally.
Exercising this needs a kernel NFS server, so it isn't covered by the
local vfstest harness; validated against a real Linux nfs-kernel-server
export over NFSv3, NFSv4.0 and NFSv4.2.
#9547
6eecd45bed