nfsd4: construct stateid from clientid and counter

Including the full clientid in the on-the-wire stateid allows more
reliable detection of bad vs. expired stateid's, simplifies code, and
ensures we won't reuse the opaque part of the stateid (as we currently
do when the same openowner closes and reopens the same file).

Signed-off-by: J. Bruce Fields <bfields@redhat.com>