hypfs: don't pin dentries twice

hypfs: don't pin dentries twice

hypfs dentries end up with refcount 2 when they are not busy.
Refcount 1 is enough to keep them pinned, and going that way
allows to simplify things nicely:
	* don't need to drop an extra reference before the
call of kill_litter_super() in ->kill_sb(); all we need
there is to reset the cleanup list - everything on it will
be taken out automatically.
	* we can make use of simple_recursive_removal() on
tree rebuilds; just make sure that only children of root
end up in the cleanup list and hypfs_delete_tree() becomes
much simpler

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>