SUNRPC/NFS: make rpc pipe upcall generic

The same function is used by idmap, gss and blocklayout code. Make it
generic.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Jim Rees <rees@umich.edu>
Cc: stable@kernel.org [3.0]
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>