Rename 'buffer' to 'data' in OSSL_PARAM

The OSSL_PARAM attribute names |buffer| and |buffer_size| may lead to
confusion, as they may make some think that the memory pointed at is
an intermediate memory are.  This is not generally the case, so we
rename |buffer| and |buffer_size| to |data| and |data_size|

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8451)