vsock/test: Adapt send_byte()/recv_byte() to handle MSG_ZEROCOPY

For a zerocopy send(), buffer (always byte 'A') needs to be preserved (thus
it can not be on the stack) or the data recv()ed check in recv_byte() might
fail.

While there, change the printf format to 0x%02x so the '\0' bytes can be
seen.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20241219-test-vsock-leaks-v4-4-a416e554d9d7@rbox.co


Signed-off-by: Jakub Kicinski <kuba@kernel.org>