torture: Fix grace-period rate output

The kvm-again.sh script relies on shell comments added to the qemu-cmd
file, but this means that code extracting values from the QEMU command in
this file must grep out those commment.  Which kvm-recheck-rcu.sh failed
to do, which destroyed its grace-period-per-second calculation.  This
commit therefore adds the needed "grep -v '^#'" to kvm-recheck-rcu.sh.

Fixes: 315957ca ("torture: Prepare for splitting qemu execution from kvm-test-1-run.sh")
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>