net: netconsole: remove msg_ready variable

Variable msg_ready is useless, since it does not represent anything. Get
rid of it, using buf directly instead.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>