x86/entry/32: Add and check a stack canary for the SYSENTER stack

The first instruction of the SYSENTER entry runs on its own tiny
stack.  That stack can be used if a #DB or NMI is delivered before
the SYSENTER prologue switches to a real stack.

We have code in place to prevent us from overflowing the tiny stack.
For added paranoia, add a canary to the stack and check it in
do_debug() -- that way, if something goes wrong with the #DB logic,
we'll eventually notice.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/6ff9a806f39098b166dc2c41c1db744df5272f29.1457578375.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>