x86/vm86: Set thread.vm86 to NULL on fork/clone

thread.vm86 points to per-task information -- the pointer should not
be copied on clone.

Fixes: d4ce0f26

 ("x86/vm86: Move fields from 'struct kernel_vm86_struct' to 'struct vm86'")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Stas Sergeev <stsp@list.ru>
Link: http://lkml.kernel.org/r/71c5d6985d70ec8197c8d72f003823c81b7dcf99.1446270067.git.luto@kernel.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>