net: netconsole: introduce variable to track body length

This new variable tracks the total length of the data to be sent,
encompassing both the message body (msgbody) and userdata, which is
collectively called body.

By explicitly defining body_len, the code becomes clearer and easier to
reason about, simplifying offset calculations and improving overall
readability of the function.

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