arm64: vdso: fix clock_getres for 4GiB-aligned res

The vdso tries to check for a NULL res pointer in __kernel_clock_getres,
but only checks the lower 32 bits as is uses CBZ on the W register the
res pointer is held in.

Thus, if the res pointer happened to be aligned to a 4GiB boundary, we'd
spuriously skip storing the timespec to it, while returning a zero error code
to the caller.

Prevent this by checking the whole pointer, using CBZ on the X register
the res pointer is held in.

Fixes: 9031fefde6f2ac1d ("arm64: VDSO support")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Andrew Pinski <apinski@cavium.com>
Reported-by: Mark Salyzyn <salyzyn@android.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/vdso/gettimeofday.S

@@ -309,7 +309,7 @@ ENTRY(__kernel_clock_getres)
 	b.ne	4f
 	ldr	x2, 6f
 2:
-	cbz	w1, 3f
+	cbz	x1, 3f
 	stp	xzr, x2, [x1]
 
 3:	/* res == NULL. */