Added another missed case to arc_summary3

It turns out that sometimes, evidently only when run inside the
ZTS handler, arc_summary3 | head > /dev/null will die with ENOTCONN,
and ruin the test run.

Added handling for that.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12160