NFSv4: Add support for the NFSv4.2 "change_attr_type" attribute

The change_attr_type allows the server to provide a description of how
the change attribute will behave. This again will allow the client to
optimise its behaviour w.r.t. attribute revalidation.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>