fscrypt: use ENOPKG when crypto API support missing

Return ENOPKG rather than ENOENT when trying to open a file that's
encrypted using algorithms not available in the kernel's crypto API.

This avoids an ambiguity, since ENOENT is also returned when the file
doesn't exist.

Note: this is the same approach I'm taking for fs-verity.
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keyinfo.c

@@ -237,11 +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)) {
-		if (PTR_ERR(tfm) == -ENOENT)
+		if (PTR_ERR(tfm) == -ENOENT) {
 			fscrypt_warn(inode,
 				     "Missing crypto API support for %s (API name: \"%s\")",
 				     mode->friendly_name, mode->cipher_str);
-		else
+			return ERR_PTR(-ENOPKG);
+		}
 		fscrypt_err(inode, "Error allocating '%s' transform: %ld",
 			    mode->cipher_str, PTR_ERR(tfm));
 		return tfm;
@@ -389,10 +390,11 @@ 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)
+			if (PTR_ERR(tfm) == -ENOENT) {
 				fscrypt_warn(NULL,
 					     "Missing crypto API support for SHA-256");
-			else
+				return -ENOPKG;
+			}
 			fscrypt_err(NULL,
 				    "Error allocating SHA-256 transform: %ld",
 				    PTR_ERR(tfm));