mm/usercopy: Detect vmalloc overruns

If you have a vmalloc() allocation, or an address from calling vmap(),
you cannot overrun the vm_area which describes it, regardless of the
size of the underlying allocation.  This probably doesn't do much for
security because vmalloc comes with guard pages these days, but it
prevents usercopy aborts when copying to a vmap() of smaller pages.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220110231530.665970-3-willy@infradead.org