xfs: remove the xfs_extent_t typedef

There are almost no users of the typedef left, kill it and switch the
remaining users to use the underlying struct.

Also fix up the comment about the struct xfs_extent definition to be
correct and read more easily.

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