xfs: pass the pag to the trace_xrep_calc_ag_resblks{,_btsize} trace points

This requires holding the pag refcount a little longer, 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>