LU-8364 ldiskfs: fixes for failover mode.

LU-8364 ldiskfs: fixes for failover mode.

when ldiskfs runs in failover mode with read-only disk,
it may loose part of allocation updates and fail while
mounting fs due to group descriptor checks before journal
replay.
don't produce panic's with on disk checks in read-only mode.

Seagate-bug-id: MRP-797
Change-Id: I54bee3a0aeb9a15f5ee2a79f7a2a2a905f19af1a
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Signed-off-by: Lokesh Nagappa Jaliminche <lokesh.jaliminche@seagate.com>
Reviewed-on: https://morpheus.xyus.xyratex.com:8443/gerrit/239
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Reviewed-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Tested-by: Alexander Lezhoev <Alexander_Lezhoev@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>
Reviewed-on: https://review.whamcloud.com/21141
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>