arc_summary3: just read /s/m/{mod}/version instead of spawning cat

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11859

cmd/arc_summary/arc_summary3

@@ -161,21 +161,11 @@ elif sys.platform.startswith('linux'):
         # The original arc_summary called /sbin/modinfo/{spl,zfs} to get
         # the version information. We switch to /sys/module/{spl,zfs}/version
         # to make sure we get what is really loaded in the kernel
-        command = ["cat", "/sys/module/{0}/version".format(request)]
-        req = request.upper()
-
-        # The recommended way to do this is with subprocess.run(). However,
-        # some installed versions of Python are < 3.5, so we offer them
-        # the option of doing it the old way (for now)
-        if 'run' in dir(subprocess):
-            info = subprocess.run(command, stdout=subprocess.PIPE,
-                                  universal_newlines=True)
-            version = info.stdout.strip()
-        else:
-            info = subprocess.check_output(command, universal_newlines=True)
-            version = info.strip()
-
-        return version
+        try:
+            with open("/sys/module/{}/version".format(request)) as f:
+                return f.read().strip()
+        except:
+            return "(unknown)"
 
     def get_descriptions(request):
         """Get the descriptions of the Solaris Porting Layer (SPL) or the