common/encrypt: support custom data unit size

common/encrypt: support custom data unit size

Make _require_scratch_encryption() and
_require_encryption_policy_support() support the new '-s' option to
set_encpolicy to specify a custom value of log2_data_unit_size.

Likewise, make _verify_ciphertext_for_encryption_policy() accept an
argument "log2_dusize=*" to cause it to use the specified data unit size
for the test and verify that the file contents are encrypted as expected
for that data unit size.

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