Handle disappearing pids in mlock_report.py

If a pid disappears on us while we're reading, we should just continue
on.

EnvironmentError is just an alias for OSError since Python 3.3, so use
the latter name. [0]

[0] https://docs.python.org/3/library/exceptions.html#OSError

Change-Id: I3a25cca328e1469f72c84a118a9691c1c0258bc4
Closes-Bug: #1926434

tools/mlock_report.py

@@ -24,8 +24,6 @@ def _get_report():
         # iterate over the /proc/%pid/status files manually
         try:
             s = open("%s/%d/status" % (psutil.PROCFS_PATH, proc.pid), 'r')
-        except EnvironmentError:
-            continue
             with s:
                 for line in s:
                     result = LCK_SUMMARY_REGEX.search(line)
@@ -35,6 +33,10 @@ def _get_report():
                             mlock_users.append({'name': proc.name(),
                                                 'pid': proc.pid,
                                                 'locked': locked})
+        except OSError:
+            # pids can disappear, we're ok with that
+            continue
+
 
     # produce a single line log message with per process mlock stats
     if mlock_users: