mips: fix vdso32 build, again

The generic vdso support adds the same #if hack in two places,
asm/vdso/vdso.h and config-n32-o32-env.c, but only the second
is actually used. The result lacks the BUILD_VDSO32_64 macro,
and that triggers a build error:

./include/linux/page-flags-layout.h:95:2: error: #error "Not enough bits in page flags"

Move the macro into the other place, and remove the duplicated
bits.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: ee38d94a ("page flags: prioritize kasan bits over last-cpuid")
Fixes: 24640f23 ("mips: Add support for generic vDSO")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org