NFS4: Fix incorrect use of sizeof() in fs/nfs/nfs4xdr.c

The XDR code should not depend on the physical allocation size of
structures like nfs4_stateid and nfs4_verifier since those may have to
change at some future date. We therefore replace all uses of
sizeof() with constants like NFS4_VERIFIER_SIZE and NFS4_STATEID_SIZE.

This also has the side-effect of fixing some warnings of the type
	format ‘%u’ expects type ‘unsigned int’, but argument X has type
		‘long unsigned int’
on 64-bit systems

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>