xfs: attach dquots before performing xfs_swap_extents

Make sure we attach dquots to both inodes before swapping their extents.
This was found via manual code inspection by looking for places where we
could call xfs_trans_mod_dquot without dquots attached to inodes, and
confirmed by instrumenting the kernel and running xfs/328.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>