NFSD: Clean up the test_stateid function

When I initially wrote it, I didn't understand how lists worked so I
wrote something that didn't use them.  I think making a list of stateids
to test is a more straightforward implementation, especially compared to
especially compared to decoding stateids while simultaneously encoding
a reply to the client.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>