fscrypt: fix race allowing rename() and link() of ciphertext dentries

Close some race conditions where fscrypt allowed rename() and link() on
ciphertext dentries that had been looked up just prior to the key being
concurrently added.  It's better to return -ENOKEY in this case.

This avoids doing the nonsensical thing of encrypting the names a second
time when searching for the actual on-disk dir entries.  It also
guarantees that DCACHE_ENCRYPTED_NAME dentries are never rename()d, so
the dcache won't have support all possible combinations of moving
DCACHE_ENCRYPTED_NAME around during __d_move().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>