net: netconsole: Add continuation line prefix to userdata messages

Add a space (' ') prefix to every userdata line to match docs for
dev-kmsg. To account for this extra character in each userdata entry,
reduce userdata entry names (directory name) from 54 characters to 53.

According to the dev-kmsg docs, a space is used for subsequent lines to
mark them as continuation lines.

> A line starting with ' ', is a continuation line, adding
> key/value pairs to the log message, which provide the machine
> readable context of the message, for reliable processing in
> userspace.

Testing for this patch::

 cd /sys/kernel/config/netconsole && mkdir cmdline0
 cd cmdline0
 mkdir userdata/test && echo "hello" > userdata/test/value
 mkdir userdata/test2 && echo "hello2" > userdata/test2/value
 echo "message" > /dev/kmsg

Outputs::

 6.8.0-rc5-virtme,12,493,231373579,-;message
  test=hello
  test2=hello2

And I confirmed all testing works as expected from the original patchset

Fixes: df03f830

 ("net: netconsole: cache userdata formatted string in netconsole_target")
Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240308002525.248672-1-thepacketgeek@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>