Increase HKDF_MAXBUF from 1024 to 2048

Increase HKDF_MAXBUF from 1024 to 2048

We've encountered some scenarios that need to use more than 1 kB of
data as the HKDF-Expand() "info" argument (which, per RFC 5869,
contains "optional context and application specific information").

Since HKDF_MAXBUF is used to size an array in the HKDF_PKEY_CTX
structure, this adds 1 kB of memory footprint to each EVP_PKEY_CTX
used for HKDF.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14649)