LU-9410 ldiskfs: no check mb bitmap if flex_bg enabled

LU-9410 ldiskfs: no check mb bitmap if flex_bg enabled

When initializes (reformat) the filesystem, the number of
free blocks in the group descriptor is calculated via the
ext2fs_reserve_super_and_bgd() (e2fsprogs). As commented
in such function: "This is not necessarily the case when
the flex_bg feature is enabled, so callers should take care!".

So it is normal that we may find the block group descriptor
that has LDISKFS_BG_BLOCK_UNINIT flag but with 0 free blocks.
The ldiskfs_mb_check_ondisk_bitmap() should NOT report error
for such block group, instead, skip the check directly.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iba0fb2bf0632a6e54222472bc724a8ea0478e9ae
Reviewed-on: https://review.whamcloud.com/28566
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>