xfs: insert the pag structures into the xarray later

Cleaning up is much easier if a structure can't be looked up yet, so only
insert the pag once it is fully set up.

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>