metadump: Add option to copy metadata blocks intact

metadump: Add option to copy metadata blocks intact

By default we zero out unused portions of metadata blocks.
This adds an option to keep blocks intact, to better investigate
corruptions.

Nothing in this patch reads it, but subsequent patches add
users of the option.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>