kselftests/ftrace: Add a test for log2 modifier of hist trigger

kselftests/ftrace: Add a test for log2 modifier of hist trigger

Add a test for log2 modifier of hist trigger in hist_mod.tc.
Here is the test result.

  ----
  # ./ftracetest test.d/trigger/trigger-hist-mod.tc
  === Ftrace unit tests ===
  [1] event trigger - test histogram modifiers	[PASS]

  # of passed:  1
  # of failed:  0
  # of unresolved:  0
  # of untested:  0
  # of unsupported:  0
  # of xfailed:  0
  # of undefined(test bug):  0
  ----

Link: http://lkml.kernel.org/r/3f1ab735c06a50b1b40d3e96b8b6a3e5ea62fd86.1457029949.git.tom.zanussi@linux.intel.com
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
Tested-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>