__d_move(): fold manipulations with ->d_child/->d_subdirs

list_del() + list_add() is a slightly pessimised list_move()
list_del() + INIT_LIST_HEAD() is a slightly pessimised list_del_init()

Interleaving those makes the resulting code even worse.  And harder to follow...

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