radix-tree: Fix UBSAN warning

get_slot_offset() can be called with a NULL 'parent' argument.
In this case, the calculated value will not be used, but calculating
it is undefined.  Rather than fixing the caller (__radix_tree_delete)
to not call get_slot_offset(), make get_slot_offset() robust against
being called with a NULL parent.

Signed-off-by: Matthew Wilcox <willy@infradead.org>