nommu: allow private mappings of read-only devices

nommu: allow private mappings of read-only devices

Slightly rearrange the logic that determines capabilities and vm_flags.
Disable BDI_CAP_MAP_DIRECT in all cases if the device can't support the
protections.  Allow private readonly mappings of readonly backing devices.

Signed-off-by: Bernd Schmidt <[email protected]>
Signed-off-by: Mike Frysinger <[email protected]>
Acked-by: David McCullough <[email protected]>
Acked-by: Greg Ungerer <[email protected]>
Acked-by: Paul Mundt <[email protected]>
Acked-by: David Howells <[email protected]>
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>