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

cmd/arc_summary/arc_summary3

@@ -42,6 +42,7 @@ import os
 import subprocess
 import sys
 import time
+import errno
 
 # We can't use env -S portably, and we need python3 -u to handle pipes in
 # the shell abruptly closing the way we want to, so...
@@ -239,6 +240,11 @@ def handle_Exception(ex_cls, ex, tb):
         # It turns out that while sys.exit() triggers an exception
         # not handled message on Python 3.8+, os._exit() does not.
         os._exit(0)
+
+    if ex_cls is OSError:
+      if ex.errno == errno.ENOTCONN:
+        sys.exit()
+
     raise ex
 
 if hasattr(sys,'unraisablehook'): # Python 3.8+