This patch changes the 'xdrsize' parameter to svc_create(), to be
     NFS4_SVC_XDRSIZE   if v4 is defined
else NFS3_SVC_XDRSIZE   if v3 is defined
else NFS2_SVC_XDRSIZE     (formerly NFSSVC_XDRSIZE)

This always works, since
     NFS4_SVC_XDRSIZE >= NFS3_SVC_XDRSIZE >= NFS2_SVC_XDRSIZE.

The value of NFSD_BUFSIZE has also been moved to const.h, since we need
the definition available in nfs4proc.c