kprobes: update jprobe_example.c for do_fork() change

In commit e80d6661

 "flagday: kill pt_regs argument of do_fork()", the
arguments to do_fork() changed.

The example code in jprobe_example.c was not updated to match, so the
arguments inside the jprobe handler do not match reality.

Fix it by updating the arguments to match do_fork(). While we're at it
use pr_info() for brevity, and print stack_start as well for interest.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>