public/io/ring.h: add FRONT/BACK_RING_ATTACH macros

public/io/ring.h: add FRONT/BACK_RING_ATTACH macros

The version of this header present in the Linux source tree has contained
such macros for some time. These macros, as the names imply, allow front
or back rings to be set up for existent (rather than freshly created and
zeroed) shared rings.

This patch is to update this, the canonical version of the header, to
match the latest definition of these macros in the Linux source.

NOTE: The way the new macros are defined allows the FRONT/BACK_RING_INIT
      macros to be re-defined in terms of them, thereby reducing
      duplication.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Juergen Gross <jgross@suse.com>