NFS: Fix another COMMIT race in pNFS

We must make sure that cinfo->ds->ncommitting is in sync with the
commit list, since it is checked as part of pnfs_commit_list().

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