setAttr left attr.Ino unset (0) and the NewInode sites left StableAttr.Ino
unset, so the kernel saw inode 0 while the node identity differed, which
breaks NFS file-handle validation. Set both to the stable VFS inode. The
bazil cmd/mount backend does not hit this because its framework assigns
stable inodes automatically; go-fuse needs them set explicitly.
Before: chmod/chown/truncate on a just-written file through an
NFS-exported mount2 mount failed with ESTALE.
After: they succeed.
Exercising the NFS handle-validation path 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
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
