ext4 crypto: handle unexpected lack of encryption keys

Fix up attempts by users to try to write to a file when they don't
have access to the encryption key.

Change-Id: I598b25bbfea3bc213130749cd39ffe32ba5fe512
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>