xattr: allow setting user.* attributes on symlinks by owner
Kvmtool and clear containers supports using user attributes to label host
files with the virtual uid/guid of the file in the container. This allows an
end user to manage their files and a complete uid space without all the ugly
namespace stuff.
The one gap in the support is symlinks because an end user can change the
ownership of a symbolic link. We support attributes on these files as you
can already (as root) set security attributes on them.
The current rules seem slightly over-paranoid and as we have a use case this
patch enables updating the attributes on a symbolic link IFF you are the
owner of the synlink (as permissions are not usually meaningful on the link
itself).
