mds: pull scatterlock of xlist in destructor
mds: pull scatterlock of xlist in destructor

Really, this should happen sooner, but for now this is equivalent to the
old xlist::item destructor.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph