Fix one timer range issue

Fix one timer range issue

According to the timer sematic, the timer can be executed at any timer
within [expires, expires_end], however, current implementation only allow
timer to be executed after expires_end, which is not conform to the timer
semantics.

This patch fix the the SPECpower score regression (~5% downgrade)
introduced by changeset 18744 "Change timer implementation to allow
variable 'slop'"

Signed-off-by: Yu Ke <ke.yu@intel.com>