selinux: fix error return code in policydb_read()

Fix to return negative error code -ENOMEM from the kvcalloc() error
handling case instead of 0, as done elsewhere in this function.

Fixes: acdf52d9 ("selinux: convert to kvmalloc")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>