Fix a failure in bio_dgram_test on the NonStop platform

Fix a failure in bio_dgram_test on the NonStop platform

The size of the datagram header is significantly larger that we might
expect on NonStop (probably driven by sizeof(BIO_ADDR)). We adjust the
size of the default buffer to take into account the header size and the
mtu.

Fixes #22013

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22058)