Quick access to stack bounds

[Roland McGrath]

> The PROT_GROWS_DOWN implementation extends the vma?  I had thought the 
> reserved address space was fixed and the flag was merely a hint for how 
> to allocate pages within that vma.  That does add a wrinkle...

Nope.  There is no reservation of the space.  i.e., any another mapping
made down there means that the stack vma can no longer extend farther down,
but instead the sp will just run into that other mapping.