crypto: xcbc - Fix incorrect error value when creating instance

If shash_alloc_instance() fails, we return the wrong error value.
This patch fixes it.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/xcbc.c b/crypto/xcbc.c
index b2cc855..3b991bf 100644
--- a/crypto/xcbc.c
+++ b/crypto/xcbc.c
@@ -252,6 +252,7 @@
 	}
 
 	inst = shash_alloc_instance("xcbc", alg);
+	err = PTR_ERR(inst);
 	if (IS_ERR(inst))
 		goto out_put_alg;