md_monitor: use correct buffer size when sending messages

The buffer size for CLI commands 'MirrorStatus' and 'MonitorStatus'
is the entire receive buffer, not just the number of bytes received.

References: bnc#814770

Signed-off-by: Hannes Reinecke <hare@suse.de>
Version 4.21

Signed-off-by: Hannes Reinecke <hare@suse.de>