xfs: convert struct typedefs in xfs_ondisk.h

Replace xfs_foo_t with struct xfs_foo where appropriate.  The next patch
will import more checks from xfs/122, and it's easier to automate
deduplication if we don't have to reason about typedefs.

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