powerpc: Avoid link stack corruption for MMU on exceptions

When we have MMU on exceptions (POWER8) and a relocatable kernel, we
need to branch from the initial exception vectors at 0x0 to up high
where the kernel might be located.  Currently we do this using the link
register.

Unfortunately this corrupts the link stack and instead we should use the
count register.  We did this for the syscall entry path in:
  6a40480 powerpc: Avoid link stack corruption in MMU on syscall entry path
but I stupidly forgot to do the same for other exceptions.

This patch changes the initial exception vectors to use the count
register instead of the link register when we need to branch up to the
relocated kernel.

I have a dodgy userspace test which loops calling a function that reads
the PVR (mfpvr in userspace will be emulated by the kernel via the
program check exception).  On POWER8 and with CONFIG_RELOCATABLE=y, I
get a ~10% performance improvement with my userspace test with this
patch.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/paca.h

@@ -93,9 +93,9 @@ struct paca_struct {
 	 * Now, starting in cacheline 2, the exception save areas
 	 */
 	/* used for most interrupts/exceptions */
-	u64 exgen[12] __attribute__((aligned(0x80)));
-	u64 exmc[12];		/* used for machine checks */
-	u64 exslb[12];		/* used for SLB/segment table misses
+	u64 exgen[13] __attribute__((aligned(0x80)));
+	u64 exmc[13];		/* used for machine checks */
+	u64 exslb[13];		/* used for SLB/segment table misses
  				 * on the linear mapping */
 	/* SLB related definitions */
 	u16 vmalloc_sllp;