ftracetest: Take the first debugfs mount found

Running ftracetests on a box that mounted debugfs in two locations
made the ftracetests fail. This is because the tests uses a grep
of debugfs from the /proc/mounts file to find the debugfs mount
point, and then appends "/tracing" to that string to get the tracing
directory.

If the debugfs directory is mounted twice, then that grep will return
two answers and appending "/tracing" to a string with two lines will
not work.

Use "head -1" to only take the first mount point found.

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>