NFSv4.1: Be conservative about the client highest slotid

If the server sends us a target that looks like an outlier, but
is lower than the existing target, then respect it anyway.
However defer actually updating the generation counter until
we get a target that doesn't look like an outlier.

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