pNFS/flexfiles: Always attempt to call layoutstats when flexfiles is enabled

Layoutstats is always desirable when using the flexfiles driver, so
we should enable it if that driver is being loaded. It is safe to do
so, because even when the mount specifies NFSv4.1, we will turn it
off if the server tells us it is unsupported.

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