Several fields in a struct rbd_dev are related to what is mapped, as
opposed to the actual base rbd image. If the base image is mapped
these are almost unneeded, but if a snapshot is mapped they describe
information about that snapshot.
In some contexts this can be a little bit confusing. So group these
mapping-related field into a structure to make it clear what they
are describing.
This also includes a minor change that rearranges the fields in the
in-core image header structure so that invariant fields are at the
top, followed by those that change.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
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