x86/idt: Fix the X86_TRAP_BP gate

Andrei Vagin reported a CRIU regression and bisected it back to:

  90f6225f ("x86/idt: Move IST stack based traps to table init")

This table init conversion loses the system-gate property of X86_TRAP_BP
and erroneously moves it from DPL3 to DPL0.

Fix it.

Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: dvlasenk@redhat.com
Cc: linux-tip-commits@vger.kernel.org
Cc: peterz@infradead.org
Cc: brgerst@gmail.com
Cc: rostedt@goodmis.org
Cc: bp@alien8.de
Cc: luto@kernel.org
Cc: jpoimboe@redhat.com
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: torvalds@linux-foundation.org
Cc: tip-bot for Jacob Shin <tipbot@zytor.com>
Link: http://lkml.kernel.org/r/20170901082630.xvyi5bwk6etmppqc@gmail.com