change kp_malloc from macro to function

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>
support function profile histogram output

add a new sample script: ftrace_histogram.kp

Now all histogram only output top 20 numbers, we must need to
limit the output numbers in histogram, otherwise the output
will flush your screen.

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>