net: netconsole: rename body to msg_body

With the introduction of the userdata concept, the term body has become
ambiguous and less intuitive.

To improve clarity, body is renamed to msg_body, making it clear that
the body is not the only content following the header.

In an upcoming patch, the term body_len will also be revised for further
clarity.

The current packet structure is as follows:

	release, header, body, [msg_body + userdata]

Here, [msg_body + userdata] collectively forms what is currently
referred to as "body." This renaming helps to distinguish and better
understand each component of the packet.

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