selftests/ftrace: Check the first record for kprobe_args_type.tc

selftests/ftrace: Check the first record for kprobe_args_type.tc

It is possible to get multiple records from trace during test and then more
than 4 arguments are assigned to ARGS.  This situation results in the failure
of kprobe_args_type.tc.  For example:
-----------------------------------------------------------
grep testprobe trace
   ftracetest-5902  [001] d... 111195.682227: testprobe: (_do_fork+0x0/0x460) arg1=334823024 arg2=334823024 arg3=0x13f4fe70 arg4=7
     pmlogger-5949  [000] d... 111195.709898: testprobe: (_do_fork+0x0/0x460) arg1=345308784 arg2=345308784 arg3=0x1494fe70 arg4=7
 grep testprobe trace
 sed -e 's/.* arg1=\(.*\) arg2=\(.*\) arg3=\(.*\) arg4=\(.*\)/\1 \2 \3 \4/'
ARGS='334823024 334823024 0x13f4fe70 7
345308784 345308784 0x1494fe70 7'
-----------------------------------------------------------

We don't care which process calls do_fork so just check the first record to
fix the issue.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>