xfs: mark dquot buffers in cache

dquot buffers always have write IO callbacks, so by marking them
directly we can avoid needing to attach ->b_iodone functions to
them. This avoids an indirect call, and makes future modifications
much simpler.

This is largely a rearrangement of the code at this point - no IO
completion functionality changes at this point, just how the
code is run is modified.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_trans_buf.c

@@ -788,5 +788,6 @@ xfs_trans_dquot_buf(
 		break;
 	}
 
+	bp->b_flags |= _XBF_DQUOTS;
 	xfs_trans_buf_set_type(tp, bp, type);
 }