mem-garnet: Fix garnet stats

This patch fix some statistics that in presence of a resetStats
instruction were not reseted. This bug makes impossible to obtain
reliable network statistics when the simulation doesn't start from tick
zero.

Change-Id: Ibec45f08d95bf0a533d94b70ec960719206ae945
Maintainer: Tushar Krishna <tushar@ece.gatech.edu>
Reviewed-on: https://gem5-review.googlesource.com/3700
Reviewed-by: Jieming Yin <bjm419@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

ruby: garnet2.0
Revamped version of garnet with more optimized single-cycle routers,
more configurability, and cleaner code.