crypto: arm64/crct10dif - add non-SIMD generic fallback

The arm64 kernel will shortly disallow nested kernel mode NEON, so
add a fallback to scalar C code that can be invoked in that case.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/arm64/crypto/crct10dif-ce-glue.c

 /*
  * Accelerated CRC-T10DIF using arm64 NEON and Crypto Extensions instructions
  *
- * Copyright (C) 2016 Linaro Ltd <ard.biesheuvel@linaro.org>
+ * Copyright (C) 2016 - 2017 Linaro Ltd <ard.biesheuvel@linaro.org>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 as
@@ -18,6 +18,7 @@
 #include <crypto/internal/hash.h>
 
 #include <asm/neon.h>
+#include <asm/simd.h>
 
 #define CRC_T10DIF_PMULL_CHUNK_SIZE	16U
 
@@ -48,9 +49,13 @@ static int crct10dif_update(struct shash_desc *desc, const u8 *data,
 	}
 
 	if (length > 0) {
-		kernel_neon_begin_partial(14);
-		*crc = crc_t10dif_pmull(*crc, data, length);
-		kernel_neon_end();
+		if (may_use_simd()) {
+			kernel_neon_begin();
+			*crc = crc_t10dif_pmull(*crc, data, length);
+			kernel_neon_end();
+		} else {
+			*crc = crc_t10dif_generic(*crc, data, length);
+		}
 	}
 
 	return 0;