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

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>
[carnil: Backport to 3.16:
 - Adjust finename change: arch/x86/kernel/entry_64.S
 - Context changes
]
[bwh: Rebase on top of "x86/traps: Enable DEBUG_STACK after cpu_init() for
 TRAP_DB/BP", and restore change in trap_init() instead of early_trap_init().
 Backport to 3.2:
 - Use zeroentry macro in entry_64.S
 - Drop changes related to breakpoint-in-NMI support
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>