AES-XTS block limit.

Limit the number of AES blocks in a data unit to 2^20 or less.
This corresponds to the mandates in IEEE Std 1619-2018 and NIST SP 800-38E.

Note: that this is a change from IEEE Std 1619-2007 which only recommended
this limit.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8627)