KEYS: Fix an error code in request_master_key()

This function has two callers and neither are able to handle a NULL
return.  Really, -EINVAL is the correct thing return here anyway.  This
fixes some static checker warnings like:

	security/keys/encrypted-keys/encrypted.c:709 encrypted_key_decrypt()
	error: uninitialized symbol 'master_key'.

Fixes: 7e70cb49 ("keys: add new key-type encrypted")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/keys/encrypted-keys/encrypted.c

@@ -437,7 +437,7 @@ static struct skcipher_request *init_skcipher_req(const u8 *key,
 static struct key *request_master_key(struct encrypted_key_payload *epayload,
 				      const u8 **master_key, size_t *master_keylen)
 {
-	struct key *mkey = NULL;
+	struct key *mkey = ERR_PTR(-EINVAL);
 
 	if (!strncmp(epayload->master_desc, KEY_TRUSTED_PREFIX,
 		     KEY_TRUSTED_PREFIX_LEN)) {