xfs: online repair of realtime bitmaps

Fix all the file metadata surrounding the realtime bitmap file, which
includes the rt geometry, file size, forks, and space mappings. The
bitmap contents themselves cannot be fixed without rt rmap, so that will
come later.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>