svc: Add xpo_prep_reply_hdr

Some transports add fields to the RPC header for replies, e.g. the TCP
record length. This function is called when preparing the reply header
to allow each transport to add whatever fields it requires.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
Acked-by: Neil Brown <neilb@suse.de>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Greg Banks <gnb@sgi.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>