net: netconsole: extract release appending into separate function

Refactor the code by extracting the logic for appending the
release into the buffer into a separate function.

The goal is to reduce the size of send_msg_fragmented() and improve
code readability.

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