evm: fix security/security_old_init_security return code

security_inode_init_security previously returned -EOPNOTSUPP, for S_PRIVATE
inodes, and relied on the callers to change it to 0.  As the callers do not
change the return code anymore, return 0, intead of -EOPNOTSUPP.

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>