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: Souvik Banerjee <souvik1997@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>