sched: panic on corrupted stack end

commit 29d6455178a09e1dc340380c582b13356227e8df upstream.

Until now, hitting this BUG_ON caused a recursive oops (because oops
handling involves do_exit(), which calls into the scheduler, which in
turn raises an oops), which caused stuff below the stack to be
overwritten until a panic happened (e.g.  via an oops in interrupt
context, caused by the overwritten CPU index in the thread_info).

Just panic directly.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[AmitP: Minor refactoring of upstream changes for linux-3.18.y]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>