nfsd: Reorder nfsd_cache_match to check more powerful discriminators first

We would normally expect the xid and the checksum to be the best
discriminators. Check them before looking at the procedure number,
etc.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>