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.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/netconsole.rst

@@ -180,7 +180,7 @@ Custom user data can be appended to the end of messages with netconsole
 dynamic configuration enabled. User data entries can be modified without
 changing the "enabled" attribute of a target.
 
-Directories (keys) under `userdata` are limited to 54 character length, and
+Directories (keys) under `userdata` are limited to 53 character length, and
 data in `userdata/<key>/value` are limited to 200 bytes::
 
  cd /sys/kernel/config/netconsole && mkdir cmdline0

drivers/net/netconsole.c

@@ -43,9 +43,11 @@ MODULE_DESCRIPTION("Console driver for network interfaces");
 MODULE_LICENSE("GPL");
 
 #define MAX_PARAM_LENGTH		256
-#define MAX_USERDATA_NAME_LENGTH	54
-#define MAX_USERDATA_VALUE_LENGTH	200
 #define MAX_USERDATA_ENTRY_LENGTH	256
+#define MAX_USERDATA_VALUE_LENGTH	200
+/* The number 3 comes from userdata entry format characters (' ', '=', '\n') */
+#define MAX_USERDATA_NAME_LENGTH	(MAX_USERDATA_ENTRY_LENGTH - \
+					MAX_USERDATA_VALUE_LENGTH - 3)
 #define MAX_USERDATA_ITEMS		16
 #define MAX_PRINT_CHUNK			1000
 
@@ -671,7 +673,7 @@ static void update_userdata(struct netconsole_target *nt)
 		 * checked to not exceed MAX items with child_count above
 		 */
 		complete_idx += scnprintf(&nt->userdata_complete[complete_idx],
-					  MAX_USERDATA_ENTRY_LENGTH, "%s=%s\n",
+					  MAX_USERDATA_ENTRY_LENGTH, " %s=%s\n",
 					  item->ci_name, udm_item->value);
 	}
 	nt->userdata_length = strnlen(nt->userdata_complete,