add a probe alias $task in trace.py

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>

tools/trace.py

@@ -204,7 +204,8 @@ class Probe(object):
                 "$gid": "(unsigned)(bpf_get_current_uid_gid() >> 32)",
                 "$pid": "(unsigned)(bpf_get_current_pid_tgid() & 0xffffffff)",
                 "$tgid": "(unsigned)(bpf_get_current_pid_tgid() >> 32)",
-                "$cpu": "bpf_get_smp_processor_id()"
+                "$cpu": "bpf_get_smp_processor_id()",
+                "$task" : "((struct task_struct *)bpf_get_current_task())"
         }
 
         def _generate_streq_function(self, string):
@@ -606,6 +607,8 @@ trace -I 'net/sock.h' \\
       'udpv6_sendmsg(struct sock *sk) (sk->sk_dport == 13568)'
         Trace udpv6 sendmsg calls only if socket's destination port is equal
         to 53 (DNS; 13568 in big endian order)
+trace -I 'linux/fs_struct.h' 'mntns_install "users = %d", $task->fs->users'
+        Trace the number of users accessing the file system of the current task
 """
 
         def __init__(self):

tools/trace_example.txt

@@ -296,4 +296,6 @@ trace -I 'net/sock.h' \\
       'udpv6_sendmsg(struct sock *sk) (sk->sk_dport == 13568)'
         Trace udpv6 sendmsg calls only if socket's destination port is equal
         to 53 (DNS; 13568 in big endian order)
+trace -I 'linux/fs_struct.h' 'mntns_install "users = %d", $task->fs->users'
+        Trace the number of users accessing the file system of the current task
 "