This changes the (undocumented, naturally) interface in disturbing ways.
Currently, *pprev will always be written to. With this change, *pprev
will only be written to if find_vma_prev() returns non-NULL.
Looking through the code, this is mostly benign. But it will cause the
CONFIG_STACK_GROWSUP version of find_extend_vma() to use an
uninitialised stack slot in ways which surely will crash the kernel.
So please have a think about that and fix it up. And please add
documentation for find_vma_prev()'s interface so we don't break it next
time.