UPSTREAM: x86/entry/64: Don't use IST entry for #BP stack

commit d8ba61ba58c88d5207c1ba2f7d9a2280e7d03be9 upstream.

There's nothing IST-worthy about #BP/int3.  We don't allow kprobes
in the small handful of places in the kernel that run at CPL0 with
an invalid stack, and 32-bit kernels have used normal interrupt
gates for #BP forever.

Furthermore, we don't allow kprobes in places that have usergs while
in kernel mode, so "paranoid" is also unnecessary.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

BUG=chromium:831269
TEST=trybots

(cherry picked from commit c6fe55e30bb6d431ee56cd4bbb582e30766c5e0e)
Signed-off-by: Daniel Wang <wonderfly@google.com>

Change-Id: I09a038a6701e956d66143aedf4ae4535f343ca2d
Reviewed-on: https://chromium-review.googlesource.com/1011388
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Commit-Queue: Daniel Wang <wonderfly@google.com>