fscrypt: improve warnings for missing crypto API support

Users of fscrypt with non-default algorithms will encounter an error
like the following if they fail to include the needed algorithms into
the crypto API when configuring the kernel (as per the documentation):

    Error allocating 'adiantum(xchacha12,aes)' transform: -2

This requires that the user figure out what the "-2" error means.
Make it more friendly by printing a warning like the following instead:

    Missing crypto API support for Adiantum (API name: "adiantum(xchacha12,aes)")

Also upgrade the log level for *other* errors to KERN_ERR.
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keyinfo.c

@@ -237,7 +237,12 @@ allocate_skcipher_for_mode(struct fscrypt_mode *mode, const u8 *raw_key,
 
 	tfm = crypto_alloc_skcipher(mode->cipher_str, 0, 0);
 	if (IS_ERR(tfm)) {
-		fscrypt_warn(inode, "Error allocating '%s' transform: %ld",
+		if (PTR_ERR(tfm) == -ENOENT)
+			fscrypt_warn(inode,
+				     "Missing crypto API support for %s (API name: \"%s\")",
+				     mode->friendly_name, mode->cipher_str);
+		else
+			fscrypt_err(inode, "Error allocating '%s' transform: %ld",
 				    mode->cipher_str, PTR_ERR(tfm));
 		return tfm;
 	}
@@ -384,8 +389,12 @@ static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
 
 		tfm = crypto_alloc_shash("sha256", 0, 0);
 		if (IS_ERR(tfm)) {
+			if (PTR_ERR(tfm) == -ENOENT)
 				fscrypt_warn(NULL,
-				     "error allocating SHA-256 transform: %ld",
+					     "Missing crypto API support for SHA-256");
+			else
+				fscrypt_err(NULL,
+					    "Error allocating SHA-256 transform: %ld",
 					    PTR_ERR(tfm));
 			return PTR_ERR(tfm);
 		}