- more fixes and comments about intent managing in ll_revalidate_it(). In few words...

- more fixes and comments about intent managing in ll_revalidate_it(). In few words, in the cases dentry is valid and no lookup will be called anymore, intent may be released by ll_intent_release() with clobering -._magic, ->it_op, etc, as it will not be used anymore. In the case revalidate failed, intent should not clobber ->magic, as it will confuse ll_frob_intent() called from ll_lookup_it(). Instead ll_drop_intent_lock() and ll_intent_free() should be used.

- in the case we found potential GNS dentry we do not cause GNS mountining by returning 0 from revalidate if GNS is disabled. This fixes possible NFS stales in some cases.