NFSv4: Split out NFS v4.2 copy completion functions

The convention in the rest of the code is to have a separate function
for anything that might be ifdef-ed out.

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