NFSv4.1: Don't decrease the value of seq_nr_highest_sent

When we're trying to figure out what the server may or may not have seen
in terms of request numbers, do not assume that requests with a larger
number were missed, just because we saw a reply to a request with a
smaller number.

Fixes: 3453d570 ("NFSv4.1: Avoid false retries when RPC calls are interrupted")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>