flexfiles: enforce per-mirror stateid only for v4 DSes

commit 320f35b7bf8cccf1997ca3126843535e1b95e9c4 upstream.

Since commit bb21ce0ad227 we always enforce per-mirror stateid.
However, this makes sense only for v4+ servers.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>