pNFS: Fix a typo in pnfs_update_layout

We're supposed to wait for the outstanding layout count to go to zero,
but that got lost somehow.

Fixes: d03360aa ("pNFS: Ensure we return the error if someone...")
Reported-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>