xfs: pass perag to xfs_alloc_read_agfl

We have the perag in most places we call xfs_alloc_read_agfl, so
pass the perag instead of a mount/agno pair.

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