swarming: use df's -l argument

appengine/swarming/swarming_bot/api/platforms/posix.py

Index: appengine/swarming/swarming_bot/api/platforms/posix.py
diff --git a/appengine/swarming/swarming_bot/api/platforms/posix.py b/appengine/swarming/swarming_bot/api/platforms/posix.py
index dca4f5e4fbc46ba52cf3ed8e5f2353999121495e..6d604e61f07ffc0d893e38f62f09c06475b2817e 100644
--- a/appengine/swarming/swarming_bot/api/platforms/posix.py
+++ b/appengine/swarming/swarming_bot/api/platforms/posix.py
@@ -12,7 +12,7 @@ import sys
 def _run_df():
   """Runs df and returns the output."""
   proc = subprocess.Popen(
-      ['/bin/df', '-k', '-P'], env={'LANG': 'C'},
+      ['/bin/df', '-k', '-P', '-l'], env={'LANG': 'C'},
       stdout=subprocess.PIPE, stderr=subprocess.PIPE)
   for l in proc.communicate()[0].splitlines():
     l = l.decode('utf-8')