powerpc: Fix comment explaining our VSID layout

We support 16TB of user address space and half a million contexts
so update the comment to reflect this.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/mmu-hash64.h

@@ -312,10 +312,9 @@ extern void slb_set_size(u16 size);
  * (i.e. everything above 0xC000000000000000), except the very top
  * segment, which simplifies several things.
  *
- * 	- We allow for 15 significant bits of ESID and 20 bits of
- * context for user addresses.  i.e. 8T (43 bits) of address space for
- * up to 1M contexts (although the page table structure and context
- * allocation will need changes to take advantage of this).
+ *	- We allow for 16 significant bits of ESID and 19 bits of
+ * context for user addresses.  i.e. 16T (44 bits) of address space for
+ * up to half a million contexts.
  *
  * 	- The scramble function gives robust scattering in the hash
  * table (at least based on some initial results).  The previous