osd: check OSDSuperblock in mkfs() when it already have superblock

osd: check OSDSuperblock in mkfs() when it already have superblock

    When we remove the ready flag from osd directory, it will execute mkfs again.
    It will read superblock but can not check osd id and cluster fsid correctly.

    It need to decode the superblock that we read from osd.
    It will always failure now, so I modify it for read osd id and fsid correctly.

http://tracker.ceph.com/issues/13586
Fixes: #13586
Signed-off-by: Vicente Cheng <freeze.bilsted@gmail.com>