ruby: Don't set the block data when a store conditional fails

ruby: Don't set the block data when a store conditional fails

Previously the Sequencer upon a Store Conditional would
unconditionally set the data of the memory location. This change
checks and prevents a failed Store Conditional from modifying any
data.

Change-Id: Id63c9579d8f054f0e95c6d338a7e31aa48762755
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2902
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>