Use kasprintf() instead of kmalloc() and snprintf().

Link: http://lkml.kernel.org/r/135a7bc36e51fd9eaa57124dd2140285b771f738.1458050835.git.geliangtang@163.comAcked-by: NNamhyung Kim <namhyung@kernel.org>
Signed-off-by: NGeliang Tang <geliangtang@163.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>