ocfs2: properly set and use inode group alloc hint

We were setting ac->ac_last_group in ocfs2_claim_suballoc_bits from
res->sr_bg_blkno.  Unfortunately, res->sr_bg_blkno is going to be zero under
normal (non-fragmented) circumstances. The discontig block group patches
effectively turned off that feature. Fix this by correctly calculating what
the next group hint should be.

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