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>

tools/testing/selftests/ftrace/ftracetest

@@ -82,7 +82,7 @@ parse_opts() { # opts
 }
 
 # Parameters
-DEBUGFS_DIR=`grep debugfs /proc/mounts | cut -f2 -d' '`
+DEBUGFS_DIR=`grep debugfs /proc/mounts | cut -f2 -d' ' | head -1`
 TRACING_DIR=$DEBUGFS_DIR/tracing
 TOP_DIR=`absdir $0`
 TEST_DIR=$TOP_DIR/test.d