nfs: update maxsz values for SETCLIENTID and EXCHANGE_ID

The spec allows for up to NFS4_OPAQUE_LIMIT (1k). While we'll almost
certainly never use that much, these ops are generally the only ones
in the compound so we might as well allow for them to be that large.

Also, the existing code didn't add in a word for the opaque length
field for either name string. Fix that while we're in there.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>