xfs: mask off the rtbitmap and summary inodes when metadir in use

Set the rtbitmap and summary file inumbers to NULLFSINO in the
superblock and make sure they're zeroed whenever we write the superblock
to disk, to mimic mkfs behavior.

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