net: netconsole: split send_msg_fragmented

Refactor the send_msg_fragmented() function by extracting the logic for
sending the message body into a new function called
send_fragmented_body().

Now, send_msg_fragmented() handles appending the release and header, and
then delegates the task of breaking up the body and sending the
fragments to send_fragmented_body().

This is the final flow now:

When send_ext_msg_udp() is called to send a message, it will:
  - call send_msg_no_fragmentation() if no fragmentation is needed
  or
  - call send_msg_fragmented() if fragmentation is needed
    * send_msg_fragmented() appends the header to the buffer, which is
      be persisted until the function returns
      * call send_fragmented_body() to iterate and populate the body of
	the message. It will not touch the header, and it will only
	replace the body, writing the msgbody and/or userdata.

Also add some comment to make the code easier to review.

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