x86: avoid dereferencing beyond stack + THREAD_SIZE

commit 60e6258cd43f9b06884f04f0f7cefb9c40f17a32 upstream

It's possible for get_wchan() to dereference past task->stack + THREAD_SIZE
while iterating through instruction pointers if fp equals the upper boundary,
causing a kernel panic.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>