metadump: clean up btree block region zeroing

metadump: clean up btree block region zeroing

Abstract out all the common operations in the btree block zeroing
so that we only have one copy of offset/len calculations and don't
require lots of casts for the pointer arithmetic.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Tested-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>