FreeBSD: Use a statement expression to implement SET_ERROR() (#16284)

This way we can avoid making assumptions about the SDT probe
implementation. No functional change intended.

Signed-off-by: Mark Johnston <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Allan Jude <[email protected]>
Reviewed-by: Rob Norris <[email protected]>
Reviewed-by: Tino Reichardt <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>