integrity: Fix memory leakage in keyring allocation error path

Key restriction is allocated in integrity_init_keyring(). However, if
keyring allocation failed, it is not freed, causing memory leaks.

Fixes: 2b6aa412 ("KEYS: Use structure to capture key restriction function and data")
Signed-off-by: GUO Zihua <guozihua@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>