DESC
remap_file_pages protection support: add VM_FAULT_SIGSEGV
EDESC
From: Ingo Molnar <mingo@elte.hu>

Since with remap_file_pages w/prot we may put PROT_NONE on a single PTE rather
than a VMA, we must handle that inside handle_mm_fault.

This value must be handled in the arch-specific fault handlers, and this
change must be ported to every arch on the world; now the new support is not
in a separate syscall, so this *must* be done unless we want stability /
security issues (the *BUG()* for unknown return values of handle_mm_fault() is
triggerable from userspace calling remap_file_pages, and on other archs, we
have VM_FAULT_OOM which is worse). However, I've alleviated this need via the
previous "safety net" patch.

This patch includes the arch-specific part for i386.

Note, however, that _proper_ support is more intrusive; we can allow a write
on a readonly VMA, but the arch fault handler currently stops that; it should
test VM_NONUNIFORM instead and call handle_mm_fault() in case it's set. And it
will have to do on its own all protection checks. This is in the following
patches.
