dentry: switch the lists of children to hlist

Saves a pointer per struct dentry and actually makes the things less
clumsy.  Cleaned the d_walk() and dcache_readdir() a bit by use
of hlist_for_... iterators.

A couple of new helpers - d_first_child() and d_next_sibling(),
to make the expressions less awful.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>