init: replace simple_strtoul with kstrtoul to improve lpj_setup

init: replace simple_strtoul with kstrtoul to improve lpj_setup

Replace simple_strtoul() with the recommended kstrtoul() for parsing the
'lpj=' boot parameter.

Check the return value of kstrtoul() and reject invalid values.  This adds
error handling while preserving existing behavior for valid values, and
removes use of the deprecated simple_strtoul() helper.

Link: https://lkml.kernel.org/r/20251122114539.446937-2-thorsten.blum@linux.dev
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>