xfs: pass the pag to the xrep_newbt_extent_class tracepoints

This requires moving a few of the callsites a little bit to ensure that
we already have the reference, but allows for the decoding to only happen
when tracing is actually enabled, and cleans up the callsites a bit.

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>