From Hugh Dickins.

radix_tree_reserve() exists solely for the tmpfs move_to_swap_cache()
and move_from_swap_cache() functions, and yet they don't need it: there
is no problem in the one page being simultaneously listed in two radix
trees (while both locks are held).  Use radix_tree_insert(), and remove
radix_tree_reserve(); also removed a few blank lines.