UBUNTU: SAUCE: sched: panic on corrupted stack end

BugLink: http://bugs.launchpad.net/bugs/1588871



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>
CVE-2016-1583
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>