ext4 crypto: check for too-short encrypted file names

An encrypted file name should never be shorter than an 16 bytes, the
AES block size.  The 3.10 crypto layer will oops and crash the kernel
if ciphertext shorter than the block size is passed to it.

Fortunately, in modern kernels the crypto layer will not crash the
kernel in this scenario, but nevertheless, it represents a corrupted
directory, and we should detect it and mark the file system as
corrupted so that e2fsck can fix this.

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