NFSv4: Don't call close if the open stateid has already been cleared

Ensure we test to see if the open stateid is actually set, before we
send a CLOSE.

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