powerpc/vdso: Fix vdso cpu truncation

The code in vdso_cpu_init that exposes the cpu and numa node to
userspace via SPRG_VDSO incorrctly masks the cpu to 12 bits. This means
that any kernel running on a box with more than 4096 threads (NR_CPUS
advertises a limit of of 8192 cpus) would expose userspace to two cpu
contexts running at the same time with the same cpu number.

Note: I'm not aware of any distro shipping a kernel with support for more
than 4096 threads today, nor of any system image that currently exceeds
4096 threads. Found via code browsing.

Fixes: 18ad51dd ("powerpc: Add VDSO version of getcpu")
Signed-off-by: NMilton Miller <miltonm@us.ibm.com>
Signed-off-by: NAnton Blanchard <anton@linux.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200715233704.1352257-1-anton@ozlabs.org

arch/powerpc/kernel/vdso.c

@@ -677,7 +677,7 @@ int vdso_getcpu_init(void)
 	node = cpu_to_node(cpu);
 	WARN_ON_ONCE(node > 0xffff);
 
-	val = (cpu & 0xfff) | ((node & 0xffff) << 16);
+	val = (cpu & 0xffff) | ((node & 0xffff) << 16);
 	mtspr(SPRN_SPRG_VDSO_WRITE, val);
 	get_paca()->sprg_vdso = val;