tracing: Fix recursive user stack trace

The user stack trace can fault when examining the trace. Which
would call the do_page_fault handler, which would trace again,
which would do the user stack trace, which would fault and call
do_page_fault again ...

Thus this is causing a recursive bug. We need to have a recursion
detector here.

[ Resubmitted by Jiri Olsa ]

[ Eric Dumazet recommended using __this_cpu_* instead of __get_cpu_* ]

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
LKML-Reference: <1289390172-9730-3-git-send-email-jolsa@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>