ANDROID: ext4: allow encrypting filenames using HEH algorithm

ANDROID: ext4: allow encrypting filenames using HEH algorithm

Update ext4 encryption to allow filenames to be encrypted using the
Hash-Encrypt-Hash (HEH) block cipher mode of operation, which is
believed to be more secure than CBC, particularly within the constant
initialization vector (IV) constraint of filename encryption.  Notably,
HEH avoids the "common prefix" problem of CBC.  Both algorithms use
AES-256 as the underlying block cipher and take a 256-bit key.

We assign mode number 126 to HEH, just below 127
(EXT4_ENCRYPTION_MODE_PRIVATE) which in some kernels is reserved for
inline encryption on MSM chipsets.  Note that these modes are not yet
upstream, which is why these numbers are being used; it's preferable to
avoid collisions with modes that may be added upstream.  Also, although
HEH is not hardware-specific, we aren't currently reserving mode number
5 for HEH upstream, since for now we are tying HEH to the new key
derivation method which might become an independent flag upstream, and
there's also a chance that details of HEH will change after it gets
wider review.

Bug: 32975945
Signed-off-by: Eric Biggers <ebiggers@google.com>
Change-Id: I81418709d47da0e0ac607ae3f91088063c2d5dd4