nfsd: nfs4callback encode_stateid helper function

To be used also for the pnfs cb_layoutrecall callback

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfsd4: fix cb_recall encoding]
    "nfsd: nfs4callback encode_stateid helper function" forgot to reserve
    more space after return from the new helper.
Reported-by: Michael Groshans <groshans@citi.umich.edu>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>