NFSv4: Fix up RCU annotations for struct nfs_netns_client

The identifier is read as an RCU protected string. Its value may
be changed during the lifetime of the network namespace by writing
a new string into the sysfs pseudofile (at which point, we free the
old string only after a call to synchronize_rcu()).

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