radix-tree: add radix_tree_gang_lookup_index

Add radix_tree_gang_lookup_index to get the radix tree
indices along with the pointers. This will be  helpful,
for e.g. if a radix_tree_delete has to be performed
depending on one of the result values.

Change-Id: Iab83d027968462aa30da5341fa3f60134b6c1137
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>