NFS: remove NFS_PAGE_TAG_LOCKED

The last real use of this tag was removed by
commit 7f2f12d9 NFS: Simplify nfs_wb_page()

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>