While processing kernel perf callchains, an bad entry can be
considered as a valid stack pointer but not as a kernel address.

In this case, we hang in an endless loop. This can happen in an
x86-32 kernel after processing the last entry in a kernel
stacktrace.

Just stop the stack frame walking after we encounter an invalid
kernel address.

This fixes a hard lockup in x86-32.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1262227945-27014-1-git-send-regression-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>