powerpc64: Fix early exit with invalid kernel SLB entries

The check for early exit should be checking the SLB entry itself.  As
currently written it was checking the address of the SLB, which is always
non-zero, so would go through the kernel SR restore loop regardless.

Submitted by:	mmacy
MFC after:	2 weeks

sys/powerpc/aim/trap_subr64.S

@@ -89,7 +89,7 @@ restore_kernsrs:
 	GET_CPUINFO(%r28)
 	addi	%r28,%r28,PC_KERNSLB
 	ld	%r29,16(%r28)		/* One past USER_SLB_SLOT */
-	cmpdi	%r28,0
+	cmpdi	%r29,0
 	beqlr				/* If first kernel entry is invalid,
 					 * SLBs not in use, so exit early */