xfs: tighten up validation of root block in inode forks

Check that root blocks that sit in the inode fork and thus have a NULL
bp don't have siblings.

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