kprobes/x86: Just return error for sanity check failure instead of using BUG_ON

Return an error from __copy_instruction() and use printk() to
give us a more productive message, since this is just an error
case which we can handle and also the BUG_ON() never tells us
why and what happened.

This is related to the following bug-report:

   https://bugzilla.redhat.com/show_bug.cgi?id=910649



Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Frank Ch. Eigler <fche@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: yrl.pp-manager.tt@hitachi.com
Link: http://lkml.kernel.org/r/20130404104230.22862.85242.stgit@mhiramat-M0-7522


Signed-off-by: Ingo Molnar <mingo@kernel.org>