blktrace: fix comment in blktrace_api.h

The `__u64 time` field of the blk_io_trace struct refers to
the time in nanoseconds, not in microseconds. It is set in
__blk_add_trace, which does the following:

    t->time = ktime_to_ns(ktime_get());

ktime_to_ns returns ktime_t in nanoseconds, not microseconds.
Signed-off-by: NSouvik Banerjee <souvik1997@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/uapi/linux/blktrace_api.h

@@ -101,7 +101,7 @@ enum blktrace_notify {
 struct blk_io_trace {
 	__u32 magic;		/* MAGIC << 8 | version */
 	__u32 sequence;		/* event number */
-	__u64 time;		/* in microseconds */
+	__u64 time;		/* in nanoseconds */
 	__u64 sector;		/* disk offset */
 	__u32 bytes;		/* transfer length */
 	__u32 action;		/* what happened */