ext4: don't allow encrypted operations without keys

[ Upstream commit 173b8439e1ba362007315868928bf9d26e5cc5a6 ]

While we allow deletes without the key, the following should not be
permitted:

total 4
-rw-r--r-- 1 root root   0 Dec 27 22:35 6,LKNRJsp209FbXoSvJWzB
-rw-r--r-- 1 root root 286 Dec 27 22:35 uRJ5vJh9gE7vcomYMqTAyD

Signed-off-by: Theodore Ts'o <tytso@mit.edu>