Impact: cleanup

Use USEC_PER_SEC and NSEC_PER_SEC instead of 1000000 and 1000000000.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <49CC7870.9000309@cn.fujitsu.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>