powerpc/sstep: Fix kernel crash if VSX is not present

emulate_step() is not checking runtime VSX feature flag before
emulating an instruction. This is causing kernel crash when kernel
is compiled with CONFIG_VSX=y but running on a machine where VSX
is not supported or disabled. Ex, while running emulate_step tests
on P6 machine:

  Oops: Exception in kernel mode, sig: 4 [#1]
  NIP [c000000000095c24] .load_vsrn+0x28/0x54
  LR [c000000000094bdc] .emulate_loadstore+0x167c/0x17b0
  Call Trace:
    0x40fe240c7ae147ae (unreliable)
    .emulate_loadstore+0x167c/0x17b0
    .emulate_step+0x25c/0x5bc
    .test_lxvd2x_stxvd2x+0x64/0x154
    .test_emulate_step+0x38/0x4c
    .do_one_initcall+0x5c/0x2c0
    .kernel_init_freeable+0x314/0x4cc
    .kernel_init+0x24/0x160
    .ret_from_kernel_thread+0x58/0xb4

With fix:
  emulate_step_test: lxvd2x         : FAIL
  emulate_step_test: stxvd2x        : FAIL

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/sstep.c

@@ -2545,6 +2545,15 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 #endif /* __powerpc64__ */
 
 	}
+
+#ifdef CONFIG_VSX
+	if ((GETTYPE(op->type) == LOAD_VSX ||
+	     GETTYPE(op->type) == STORE_VSX) &&
+	    !cpu_has_feature(CPU_FTR_VSX)) {
+		return -1;
+	}
+#endif /* CONFIG_VSX */
+
 	return 0;
 
  logical_done: