tracing: Change the function format to display function names by perf

Here is an example for this change.

$ sudo perf record -e 'ftrace:function' --filter='ip==schedule'
$ sudo perf report

The output of perf before this patch:

\# Samples: 100  of event 'ftrace:function'
\# Event count (approx.): 100
\#
\# Overhead  Trace output
\# ........  ......................................
\#
    51.00%   ffffffff81f6aaa0 <-- ffffffff81158e8d
    29.00%   ffffffff81f6aaa0 <-- ffffffff8116ccb2
     8.00%   ffffffff81f6aaa0 <-- ffffffff81f6f2ed
     4.00%   ffffffff81f6aaa0 <-- ffffffff811628db
     4.00%   ffffffff81f6aaa0 <-- ffffffff81f6ec5b
     2.00%   ffffffff81f6aaa0 <-- ffffffff81f6f21a
     1.00%   ffffffff81f6aaa0 <-- ffffffff811b04af
     1.00%   ffffffff81f6aaa0 <-- ffffffff8143ce17

After this patch:

\# Samples: 36  of event 'ftrace:function'
\# Event count (approx.): 36
\#
\# Overhead  Trace output
\# ........  ............................................
\#
    38.89%   schedule <-- schedule_hrtimeout_range_clock
    27.78%   schedule <-- worker_thread
    13.89%   schedule <-- schedule_timeout
    11.11%   schedule <-- smpboot_thread_fn
     5.56%   schedule <-- rcu_gp_kthread
     2.78%   schedule <-- exit_to_usermode_loop

Link: http://lkml.kernel.org/r/20190209161919.32350-1-changbin.du@gmail.com

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>