fbmem: VM_IO set, but not propagated

fbmem: VM_IO set, but not propagated

When we setup up the VMA flags for the mmap flag and we end up using the
fallback mmap functionality we set the vma->vm_flags |= VM_IO.  However we
neglect to propagate the flag to the vma->vm_page_prot.

This bug was found when Linux kernel was running under Xen.  In that
scenario, any page that has VM_IO flag to it, means that it MUST be a
MMIO/VRAM backend memory , _not_ System RAM.  That is what the fbmem.c
does: sets VM_IO, ioremaps the region - everything is peachy.

Well, not exactly.  The vm_page_prot does not get the relevant PTE flags
set (_PAGE_IOMAP) which under Xen is a death-kneel to pages that are
referencing real physical devices but don't have that flag set.

This patch fixes this.

Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: Daniel De Graaf <[email protected]>
Tested-by: Eamon Walsh <[email protected]>
Cc: Florian Tobias Schandinat <[email protected]>
Cc: Dave Airlie <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>