xfs: track an active perag reference in filestreams

Rather than just track the agno of the reference, track a referenced
perag pointer instead. This will allow active filestreams to prevent
AGs from going away until the filestreams have been torn down.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>