powerpc/sstep: Fix darn emulation

Commit 8813ff49 ("powerpc/sstep: Check instruction validity
against ISA version before emulation") introduced a proper way to skip
unknown instructions. This makes sure that the same is used for the
darn instruction when the range selection bits have a reserved value.

Fixes: a23987ef ("powerpc: sstep: Add support for darn instruction")
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210204080744.135785-2-sandipan@linux.ibm.com

arch/powerpc/lib/sstep.c

@@ -1916,7 +1916,7 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 				goto compute_done;
 			}
 
-			return -1;
+			goto unknown_opcode;
 #ifdef __powerpc64__
 		case 777:	/* modsd */
 			if (!cpu_has_feature(CPU_FTR_ARCH_300))