[S390] Cleanup struct _lowcore usage and defines.

Use asm offsets to make sure the offset defines to struct _lowcore and
its layout don't get out of sync.
Also add a BUILD_BUG_ON() which checks that the size of the structure
is sane.
And while being at it change those sites which use odd casts to access
the current lowcore. These should use S390_lowcore instead.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/cio.c

@@ -625,8 +625,8 @@ void __irq_entry do_IRQ(struct pt_regs *regs)
 	/*
 	 * Get interrupt information from lowcore
 	 */
-	tpi_info = (struct tpi_info *) __LC_SUBCHANNEL_ID;
-	irb = (struct irb *) __LC_IRB;
+	tpi_info = (struct tpi_info *)&S390_lowcore.subchannel_id;
+	irb = (struct irb *)&S390_lowcore.irb;
 	do {
 		kstat_cpu(smp_processor_id()).irqs[IO_INTERRUPT]++;
 		/*
@@ -682,10 +682,10 @@ static int cio_tpi(void)
 	struct irb *irb;
 	int irq_context;
 
-	tpi_info = (struct tpi_info *) __LC_SUBCHANNEL_ID;
+	tpi_info = (struct tpi_info *)&S390_lowcore.subchannel_id;
 	if (tpi(NULL) != 1)
 		return 0;
-	irb = (struct irb *) __LC_IRB;
+	irb = (struct irb *)&S390_lowcore.irb;
 	/* Store interrupt response block to lowcore. */
 	if (tsch(tpi_info->schid, irb) != 0)
 		/* Not status pending or not operational. */
@@ -885,7 +885,7 @@ __clear_io_subchannel_easy(struct subchannel_id schid)
 		struct tpi_info ti;
 
 		if (tpi(&ti)) {
-			tsch(ti.schid, (struct irb *)__LC_IRB);
+			tsch(ti.schid, (struct irb *)&S390_lowcore.irb);
 			if (schid_equal(&ti.schid, &schid))
 				return 0;
 		}
@@ -1083,7 +1083,7 @@ int __init cio_get_iplinfo(struct cio_iplinfo *iplinfo)
 	struct subchannel_id schid;
 	struct schib schib;
 
-	schid = *(struct subchannel_id *)__LC_SUBCHANNEL_ID;
+	schid = *(struct subchannel_id *)&S390_lowcore.subchannel_id;
 	if (!schid.one)
 		return -ENODEV;
 	if (stsch(schid, &schib))