NFS: Fix a missing initialisation when reading the SELinux label

Ensure that _nfs4_do_get_security_label() also initialises the
SEQUENCE call correctly, by having it call into nfs4_call_sync().

Reported-by: Jeff Layton <jlayton@redhat.com>
Cc: stable@vger.kernel.org # 3.11+
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>