xfs: constify pag arguments to trace points

Trace points never modify their arguments.  Mark all the pag objects
passed to trace points.  The exception is the xfs_ag_resv_class, which
uses the xfs_perag_resv helper that can't be marked const due to
other users modifying the returned structure.

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>