security: move LSM xattrnames to xattr.h

Make the security extended attributes names global. Updated to move
the remaining Smack xattrs.

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>