NFS: Merge nfs41_free_stateid() with _nfs41_free_stateid()

Having these exist as two functions doesn't seem to add anything useful,
and I think merging them together makes this easier to follow.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>