[XFS] Reduce size of xfs_trans_t structure. * remove ->t_forw, ->t_back --

unused * ->t_ag_freeblks_delta, ->t_ag_flist_delta, ->t_ag_btree_delta
are debugging aid -- wrap them in everyone's favourite way.    As a
result, cut "xfs_trans" slab object size from 592 to 572 bytes here.

SGI-PV: 904196
SGI-Modid: xfs-linux-melb:xfs-kern:26319a

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>