After calling radix_tree_iter_retry(), 'slot' will be set to NULL. This
can cause radix_tree_next_slot() to dereference the NULL pointer. Add
Konstantin Khlebnikov's test to the regression framework.
Signed-off-by: Matthew Wilcox
Reported-by: Konstantin Khlebnikov
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds