bfq: bfq_check_waker() should be static

It's only used in the same file, mark is appropriately static.

Fixes: 71217df3

 ("block, bfq: make waker-queue detection more robust")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>