security: have cap_dentry_init_security return error

Currently, cap_dentry_init_security returns 0 without actually
initializing the security label. This confuses its only caller
(nfs4_label_init_security) which expects an error in that situation, and
causes it to end up sending out junk onto the wire instead of simply
suppressing the label in the attributes sent.

When CONFIG_SECURITY is disabled, security_dentry_init_security returns
-EOPNOTSUPP. Have cap_dentry_init_security do the same.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>