xfs: factor out rtbitmap/summary initialization helpers

xfs: factor out rtbitmap/summary initialization helpers

Add helpers to libxfs that can be shared by growfs and mkfs for
initializing the rtbitmap and summary, and by passing the optional data
pointer also by repair for rebuilding them.  This will become even more
useful when the rtgroups feature adds a metadata header to each block,
which means even more shared code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
[djwong: minor documentation and data advance tweaks]
Signed-off-by: Darrick J. Wong <djwong@kernel.org>