bcache: trace missed reading by cache_missed

Missed reading IOs are identified by s->cache_missed, not the
s->cache_miss, so in trace_bcache_read() using trace_bcache_read
to identify whether the IO is missed or not.

Signed-off-by: Tang Junhui <tang.junhui.linux@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>