sched: panic on corrupted stack end

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.

Conflicts:
	kernel/sched/core.c

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dennis Cagle <d-cagle@codeaurora.org>
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
Git-commit: 29d6455178a09e1dc340380c582b13356227e8df
(cherry picked from commit 29d6455178a09e1dc340380c582b13356227e8df)
Change-Id: I73409be3e4cfba82bae36a487227eb5260cd6e37