ocfs2: Use the right group in nfs sync check.

We have added discontig block group now, and now an inode
can be allocated in an discontig block group. So get
it in ocfs2_get_suballoc_slot_bit.

The old ocfs2_test_suballoc_bit gets group block no
from the allocation inode which is wrong. Fix it by
passing the right group.

Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Tao Ma <tao.ma@oracle.com>