x86/mpx/selftests: Fix up weird arrays

The MPX hardware data structurse are defined in a weird way: they define
their size in bytes and then union that with the type with which we want
to access them.

Yes, this is weird, but it does work.  But, new GCC's complain that we
are accessing the array out of bounds.  Just make it a zero-sized array
so gcc will stop complaining.  There was not really a bug here.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20171111001229.58A7933D@viggo.jf.intel.com


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