x86/ldt: Correct LDT access in single stepping logic

Commit 37868fe1 ("x86/ldt: Make modify_ldt synchronous")
introduced a new struct ldt_struct anchored at mm->context.ldt.

convert_ip_to_linear() was changed to reflect this, but indexing
into the ldt has to be changed as the pointer is no longer void *.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: <stable@vger.kernel.org> # On top of: 37868fe1: x86/ldt: Make modify_ldt synchronous
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@suse.de
Link: http://lkml.kernel.org/r/1438848278-12906-1-git-send-email-jgross@suse.com


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