linux: fix condition for entering tickless state

linux: fix condition for entering tickless state

I noticed that timer interrupts were running at full rate regardless
of whether the system was idle.
softlockup_get_next_event() must return the maximum of zero and the
delta to the next event, not the minimum (which is always zero or
negative, yielding zero or a very large positive number as return
value).

Signed-off-by: Jan Beulich <jbeulich@novell.com>