tick-sched: Fix mismerge

This was a remnant of e2751777059f (rcu: Precompute RCU_FAST_NO_HZ
timer offsets), which was undone, along with other RCU patches, during
the LP kernel merge. Fixes the softirq storm.

Change-Id: I2583ab2225ff5fb1fac3e6d3908113cc9d2e6a91