MRP-691 ldiskfs: hold bitmaps to minimize reads during writes

The patch makes bitmap pages stay in correct LRUs to avoid
unnecessary page eviction and possible bitmap read during
bulk writes.

Change-Id: I70c2f44707a97822a2f9b13e74fae19753d50792
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/135
Tested-by: Jenkins
Tested-by: Alexander Lezhoev <Alexander_Lezhoev@xyratex.com>
Reviewed-by: Alexander Boyko <Alexander_Boyko@xyratex.com>
Reviewed-by: Alexander Zarochentsev <alexander_zarochentsev@xyratex.com>
Reviewed-by: Nathan Rutman <Nathan_Rutman@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>