blk-iocost: tracing: atomic64_read(&ioc->vtime_rate) is assigned an extra semicolon

Remove extra semicolon.

Link: https://lkml.kernel.org/r/20220629030013.10362-1-kunyu@nfschina.com

Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Li kunyu <kunyu@nfschina.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>