xfs_io/encrypt: support specifying crypto data unit size

xfs_io/encrypt: support specifying crypto data unit size

Add an '-s' option to the 'set_encpolicy' command of xfs_io to allow
exercising the log2_data_unit_size field that is being added to struct
fscrypt_policy_v2 (kernel patch:
https://lore.kernel.org/linux-fscrypt/20230925055451.59499-6-ebiggers@kernel.org).

The xfs_io support is needed for xfstests
(https://lore.kernel.org/fstests/20231013061403.138425-1-ebiggers@kernel.org),
which currently relies on xfs_io to access the encryption ioctls.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>