crypto: arm/chacha - fix build failured when kernel mode NEON is disabled

When the ARM accelerated ChaCha driver is built as part of a configuration
that has kernel mode NEON disabled, we expect the compiler to propagate
the build time constant expression IS_ENABLED(CONFIG_KERNEL_MODE_NEON) in
a way that eliminates all the cross-object references to the actual NEON
routines, which allows the chacha-neon-core.o object to be omitted from
the build entirely.

Unfortunately, this fails to work as expected in some cases, and we may
end up with a build error such as

  chacha-glue.c:(.text+0xc0): undefined reference to `chacha_4block_xor_neon'

caused by the fact that chacha_doneon() has not been eliminated from the
object code, even though it will never be called in practice.

Let's fix this by adding some IS_ENABLED(CONFIG_KERNEL_MODE_NEON) tests
that are not strictly needed from a logical point of view, but should
help the compiler infer that the NEON code paths are unreachable in
those cases.

Fixes: b36d8c09 ("crypto: arm/chacha - remove dependency on generic ...")
Reported-by: Russell King <linux@armlinux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/arm/crypto/chacha-glue.c

@@ -115,7 +115,7 @@ static int chacha_stream_xor(struct skcipher_request *req,
 		if (nbytes < walk.total)
 			nbytes = round_down(nbytes, walk.stride);
 
-		if (!neon) {
+		if (!IS_ENABLED(CONFIG_KERNEL_MODE_NEON) || !neon) {
 			chacha_doarm(walk.dst.virt.addr, walk.src.virt.addr,
 				     nbytes, state, ctx->nrounds);
 			state[12] += DIV_ROUND_UP(nbytes, CHACHA_BLOCK_SIZE);
@@ -159,7 +159,7 @@ static int do_xchacha(struct skcipher_request *req, bool neon)
 
 	chacha_init_generic(state, ctx->key, req->iv);
 
-	if (!neon) {
+	if (!IS_ENABLED(CONFIG_KERNEL_MODE_NEON) || !neon) {
 		hchacha_block_arm(state, subctx.key, ctx->nrounds);
 	} else {
 		kernel_neon_begin();