xdrgen: Address some checkpatch whitespace complaints

xdrgen: Address some checkpatch whitespace complaints

This is a roll-up of three template fixes that eliminate noise from
checkpatch output so that it's easier to spot non-trivial problems.

To follow conventional kernel C style, when a union declaration is
marked with "pragma public", there should be a blank line between
the emitted "union xxx { ... };" and the decoder and encoder
function declarations.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>