MRP-797 ldiskfs: fixes for failover mode.

when ldiskfs run in failover mode with r/o disk, it may lost part of
allocation updates and fail with mounting fs due gd checks before journal
replay.
don't produce a panic's with on disk checks in r/o mode.

Change-Id: I59c3b5f225dacccee23e68d0ee0be6667df061e2
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/241
Tested-by: Alexander Lezhoev <Alexander_Lezhoev@xyratex.com>
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Reviewed-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>