netconsole: Warn if MAX_USERDATA_ITEMS limit is exceeded

netconsole: Warn if MAX_USERDATA_ITEMS limit is exceeded

netconsole configfs helpers doesn't allow the creation of more than
MAX_USERDATA_ITEMS items.

Add a warning when netconsole userdata update function attempts sees
more than MAX_USERDATA_ITEMS entries.

Replace silent ignore mechanism with WARN_ON_ONCE() to highlight
potential misuse during development and debugging.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250108-netcons_overflow_test-v3-1-3d85eb091bec@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>