A custom allocator without __GFP_COMP that copies to userspace has been
found in vmw_execbuf_process[1], so this disables the page-span checker
by placing it behind a CONFIG for future work where such things can be
tracked down later.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1373326Reported-by: NVinson Lee <vlee@freedesktop.org>
Fixes: f5509cc1 ("mm: Hardened usercopy")
Signed-off-by: NKees Cook <keescook@chromium.org>