xfs: pass perag to xfs_read_agi

We have the perag in most palces we call xfs_read_agi, 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>