security: insufficient range checks in certain fault handlers

security: insufficient range checks in certain fault handlers

This is the 2.4 version of Nick Piggin's work on 2.6 fault handlers.
This deals with security vulnerability CVE-2008-0007.

Drivers that register a ->nopage handler, that does not range-check its
offset argument, must set VM_DONTEXPAND in the vm_flags to ensure the
offset is within bounds.

Signed-off-by: Willy Tarreau <w@1wt.eu>