nfsd: fix filecache lookup

If the lookup keeps finding a nfsd_file with an unhashed open file,
then retry once only.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org
Fixes: 65294c1f

 "nfsd: add a new struct file caching facility to nfsd"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>