powerpc/mm: use macro PGTABLE_EADDR_SIZE instead of digital

In case of extending the eaddr in future, use this macro
PGTABLE_EADDR_SIZE to ease the maintenance of the code.
Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/slb_low.S

@@ -35,7 +35,7 @@ _GLOBAL(slb_allocate_realmode)
 	 * check for bad kernel/user address
 	 * (ea & ~REGION_MASK) >= PGTABLE_RANGE
 	 */
-	rldicr. r9,r3,4,(63 - 46 - 4)
+	rldicr. r9,r3,4,(63 - PGTABLE_EADDR_SIZE - 4)
 	bne-	8f
 
 	srdi	r9,r3,60		/* get region */