checkpatch: Do not allow deprecated g_memdup()

g_memdup() is insecure and as been deprecated in GLib 2.68.
QEMU provides the safely equivalent g_memdup2() wrapper.

Do not allow more g_memdup() calls in the repository, provide
a hint to use g_memdup2().

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210903174510.751630-29-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>