Revert "LU-1234 dcache: don't drop invalid dentry arbitrarily"

This reverts commit 2d91e7b9080df12b9afc59babc9c4c6022e21907

Change-Id: I827961c4cca7553d2255558479bb3e0810b6eacc
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/889
Tested-by: Jenkins
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>