ceph: drop "_cb" from name of struct ceph_vxattr_cb

A struct ceph_vxattr_cb does not represent a callback at all, but
rather a virtual extended attribute itself.  Drop the "_cb" suffix
from its name to reflect that.

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>