s390/cert_store: fix error return code in fill_cs_keyring()

The 'rc' will be re-assigned to 0 after calling get_vcssb(), it
needs be set to error code if create_cs_keyring() fails.

[hca@linux.ibm.com: slightly changed coding style]
Fixes: 8cf57d72 ("s390: add support for user-defined certificates")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20230728084228.3186083-1-yangyingliang@huawei.com


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>