block/blk-throttle: silence !BLK_DEV_IO_TRACE variable warnings

If blk-throttle is enabled but blktrace is not, then the compiler will
notice that the following two variables are unused:

../block/blk-throttle.c: In function 'throtl_pending_timer_fn':
../block/blk-throttle.c:1153:30: warning: unused variable 'bio_cnt_w' [-Wunused-variable]
 1153 |                 unsigned int bio_cnt_w = sq_queued(sq, WRITE);
      |                              ^~~~~~~~~
../block/blk-throttle.c:1152:30: warning: unused variable 'bio_cnt_r' [-Wunused-variable]
 1152 |                 unsigned int bio_cnt_r = sq_queued(sq, READ);
      |                              ^~~~~~~~~

Silence that my annotating them with __maybe_unused.

Fixes: 28ad83b7 ("blk-throttle: Split the service queue")
Link: https://lore.kernel.org/all/20250515130830.9671-1-aishwarya.tcv@arm.com/


Reported-by: Aishwarya <aishwarya.tcv@arm.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>