xfs: kill xfs_qm_idtodq

This function doesn't help the code flow, so merge the dquot allocation and
transaction handling into xfs_qm_dqread.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>