fs: Replace simple_strtoul with kstrtoul in set_ihash_entries

fs: Replace simple_strtoul with kstrtoul in set_ihash_entries

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

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

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20251218112144.225301-2-thorsten.blum@linux.dev
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>