nfs/localio: avoid bouncing LOCALIO if nfs_client_is_local()

Previously nfs_local_probe() was made to disable and then attempt to
re-enable LOCALIO (via LOCALIO protocol handshake) if/when it was
called and LOCALIO already enabled.

Vague memory for _why_ this was the case is that this was useful
if/when a local NFS server were to be restarted with a local NFS
client connected to it.

But as it happens this causes an absurd amount of LOCALIO flapping
which has a side-effect of too much IO being needlessly sent to NFSD
(using RPC over the loopback network interface).  This is the
definition of "serious performance loss" (that negates the point of
having LOCALIO).

So remove this mis-optimization for re-enabling LOCALIO if/when an NFS
server is restarted (which is an extremely rare thing to do).  Will
revisit testing that scenario again but in the meantime this patch
restores the full benefit of LOCALIO.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>