The $task refers to the current task. In my particular case, I need
to trace the number of users for file system associated with the current
task. With the probe alias $task, trace.py can easily trace this event

  ......
  trace.py -I 'linux/fs_struct.h' 'mntns_install "users = %d", $task->fs->users'
  PID     TID     COMM            FUNC             -
  176566  176566  python2.7       mntns_install    users = 2
  176566  176566  python2.7       mntns_install    users = 2
  ......

With $task probe alias, kernel tast_struct fields can be used in trace.py
filter or output easily even if they cannot be accessed through input
parameters.
Signed-off-by: Yonghong Song <yhs@fb.com>