cgroup fs: avoid switching ->d_op on live dentry

Switching d_op on a live dentry is racy in general, so avoid it. In this case
it is a negative dentry, which is safer, but there are still concurrent ops
which may be called on d_op in that case (eg. d_revalidate). So in general
a filesystem may not do this. Fix cgroupfs so as not to do this.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>