LU-7304 ldiskfs: fix bug when bigalloc is enabled

LU-7304 ldiskfs: fix bug when bigalloc is enabled

See following error when enabled bigalloc feature
for ldiskfs rhel7:

LDISKFS-fs error (device sdb):
ldiskfs_mb_check_ondisk_bitmap:3611: comm mkdir:
 on-disk bitmap for group 8corrupted: 0 blocks free in
 bitmap, 32768 - in gd

Fixed to use EXT4_CLUSTERS_PER_GROUP, otherwise,
we will get wrong value and fail to check, which
make FS become RO..

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I7f61918918e6f4e2f372929181b704b0648dcbca
Reviewed-on: http://review.whamcloud.com/16832
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>