fs/nfs/nfs3proc.c is making use of xattr but was getting linux/xattr.h
indirectly through linux/cgroup.h, which will soon drop the inclusion
of xattr.h. Explicitly include linux/xattr.h from nfs3proc.c so that
compilation doesn't fail when linux/cgroup.h drops linux/xattr.h.
As the following cgroup changes will depend on these changes, it
probably would be easier to route this through cgroup branch. Would
that be okay?