Fix min, max times in /proc/lock_stats

(1) When collecting lock hold and wait times, if the current minimum
    time is zero, it will be replaced by the next time.

(2) When aggregating minimum and maximum lock hold and wait times
    accross cpus, the values are added, instead of selecting the
    minimum and maximum.
Signed-off-by: NFrank Rowand <frank.rowand@am.sony.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <4B05BBAE.2050005@am.sony.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>