block: Add child access functions

block: Add child access functions

There are BDS children that the general block layer code can access,
namely bs->file and bs->backing.  Since the introduction of filters and
external data files, their meaning is not quite clear.  bs->backing can
be a COW source, or it can be a filtered child; bs->file can be a
filtered child, it can be data and metadata storage, or it can be just
metadata storage.

This overloading really is not helpful.  This patch adds functions that
retrieve the correct child for each exact purpose.  Later patches in
this series will make use of them.  Doing so will allow us to handle
filter nodes in a meaningful way.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>