cifsd: fix memdup.cocci warnings

fs/cifsd/smb2pdu.c:1177:27-34: WARNING opportunity for kmemdup

 Use kmemdup rather than duplicating its implementation

Generated by: scripts/coccinelle/api/memdup.cocci
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifsd/smb2pdu.c

@@ -1161,13 +1161,11 @@ static int alloc_preauth_hash(struct ksmbd_session *sess,
 	if (sess->Preauth_HashValue)
 		return 0;
 
-	sess->Preauth_HashValue = kmalloc(PREAUTH_HASHVALUE_SIZE, GFP_KERNEL);
+	sess->Preauth_HashValue = kmemdup(conn->preauth_info->Preauth_HashValue,
+			PREAUTH_HASHVALUE_SIZE, GFP_KERNEL);
 	if (!sess->Preauth_HashValue)
 		return -ENOMEM;
 
-	memcpy(sess->Preauth_HashValue,
-	       conn->preauth_info->Preauth_HashValue,
-	       PREAUTH_HASHVALUE_SIZE);
 	return 0;
 }