Add error message when sha256 algorithm is not supported

It failed silently when crypto_alloc_tfm() failed, so add an error
message to inform the developer to enable sha256 algorithm support.

Signed-off-by: Yu Chien Peter Lin <peterlin.tw@pm.me>

examples/cryptosha256.c

@@ -26,8 +26,12 @@ static int cryptosha256_init(void)
     struct shash_desc *shash;
 
     sha256 = crypto_alloc_shash("sha256", 0, 0);
-    if (IS_ERR(sha256))
+    if (IS_ERR(sha256)) {
+        pr_err(
+            "%s(): Failed to allocate sha256 algorithm, enable CONFIG_CRYPTO_SHA256 and try again.\n",
+            __func__);
         return -1;
+    }
 
     shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(sha256),
                     GFP_KERNEL);

lkmpg.tex

@@ -1891,6 +1891,7 @@ To handle crypto stuff, the kernel has its own API enabling common methods of en
 
 Calculating and checking the hashes of things is a common operation.
 Here is a demonstration of how to calculate a sha256 hash within a kernel module.
+To provide the sha256 algorithm support, make sure \cpp|CONFIG_CRYPTO_SHA256| is enabled in kernel.
 
 \samplec{examples/cryptosha256.c}