ANDROID: fscrypt: set dun_bytes more precisely

Make fscrypt set dun_bytes to only what it actually needs, so that it
can make use of inline crypto hardware in more cases.

Bug: 144046242
Bug: 153512828
Change-Id: I36f90ea6b64ef51a9d58ffb069d2cba74965c239
Signed-off-by: Eric Biggers <ebiggers@google.com>