nfs: machine credential support for additional operations

Allow LAYOUTRETURN and DELEGRETURN to use machine credentials if the
server supports it. Add request for OPEN_DOWNGRADE as the close path
also uses that.

Signed-off-by: Andrew Elble <aweits@rit.edu>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>