NFSv4: Track the number of referring calls in struct cb_process_state

When the server gives us a set of referring calls, to tell us that the
NFSv4.1 callback needs to be ordered with respect to those calls, then
we may want to make that information available to the operations. In
certain cases, it may allow them to optimise their behaviour due to the
extra knowledge.

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