Starting in Linux 5.10, trying to write to /dev/{null,zero} errors out.
Prefer to inform people when this happens rather than hoping they guess
what's wrong.
Reviewed-by: Antonio Russo <aerusso@aerusso.net> Reviewed-by: Ahelenia ZiemiaĆska <nabijaczleweli@nabijaczleweli.xyz> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: John Kennedy <john.kennedy@delphix.com> Signed-off-by: Rich Ercolani <rincebrain@gmail.com> Closes: #11991