[PATCHES] cls_u32 cleanups and fixes

[Al Viro]

        Several cls_u32 patches: fixing refcounting, preventing
links to and deletion of root hnodes, validating divisor.  Plus
cleanups - removal of some useless fields and saner handling of
tc_u_common hashtable.  The first 3 in series are fixes (and
-stable fodder), the rest - cleanups.  Branch can be found in
vfs.git#misc.cls_u32; individual patches will go in followups.