NFS: Force the legacy idmapper to be single threaded

commit b1027439dff844675f6c0df97a1b1d190791a699 upstream.

It was initially coded under the assumption that there would only be one
request at a time, so use a lock to enforce this requirement..

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>