ocfs2: Set ac_last_group properly with discontig group.

ac_last_group is used to record the last block group we
used during allocation. But the initialization process
only calls ocfs2_which_suballoc_group and fails to
use suballoc_loc properly. So let us do it.
Another function ocfs2_test_suballoc_bit also needs fix.

I have searched all the callers of ocfs2_which_suballoc_group,
and all the callers notices suballoc_loc now.

Signed-off-by: Tao Ma <tao.ma@oracle.com>