fscrypt-crypt-util: rename block to data unit

fscrypt-crypt-util: rename block to data unit

Rename the --block-size option to --data-unit-size, and rename the
--block-number option to --data-unit-index.

This does not change any functionality, but this avoids confusion now
that the kernel supports the case where the crypto data unit size is not
the same as the filesystem block size.  fscrypt-crypt-util cares about
the crypto data unit size, not the filesystem block size.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>