dcache: Replace simple_strtoul with kstrtoul in set_dhash_entries

dcache: Replace simple_strtoul with kstrtoul in set_dhash_entries

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

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

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20251216145236.44520-2-thorsten.blum@linux.dev
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>