powerpc/math-emu: Fix load/store indexed emulation

Load/store indexed instructions where the index register RA=R0, such
as "lfdx f1,0,r3", are not illegal.

Load/store indexed with update instructions where the index register
RA=R0, such as "lfdux f1,0,r3", are invalid, and, to be consistent
with existing math-emu behavior for other invalid instruction forms,
will signal as illegal.
Signed-off-by: NJames Yang <James.Yang@freescale.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/math-emu/math.c

@@ -380,21 +380,16 @@ do_mathemu(struct pt_regs *regs)
 	case XE:
 		idx = (insn >> 16) & 0x1f;
 		op0 = (void *)&current->thread.TS_FPR((insn >> 21) & 0x1f);
-		if (!idx) {
-			if (((insn >> 1) & 0x3ff) == STFIWX)
-				op1 = (void *)(regs->gpr[(insn >> 11) & 0x1f]);
-			else
-				goto illegal;
-		} else {
-			op1 = (void *)(regs->gpr[idx] + regs->gpr[(insn >> 11) & 0x1f]);
-		}
-
+		op1 = (void *)((idx ? regs->gpr[idx] : 0)
+				+ regs->gpr[(insn >> 11) & 0x1f]);
 		break;
 
 	case XEU:
 		idx = (insn >> 16) & 0x1f;
+		if (!idx)
+			goto illegal;
 		op0 = (void *)&current->thread.TS_FPR((insn >> 21) & 0x1f);
-		op1 = (void *)((idx ? regs->gpr[idx] : 0)
+		op1 = (void *)(regs->gpr[idx]
 				+ regs->gpr[(insn >> 11) & 0x1f]);
 		break;