nfsd4: fix up replay_matches_cache()

When running an nfs stress test, I see quite a few cached replies that
don't match up with the actual request.  The first comment in
replay_matches_cache() makes sense, but the code doesn't seem to
match... fix it.

This isn't exactly a bugfix, as the server isn't required to catch every
case of a false retry.  So, we may as well do this, but if this is
fixing a problem then that suggests there's a client bug.

Fixes: 53da6a53 ("nfsd4: catch some false session retries")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>