sbitmap: trivial - update comment for sbitmap_deferred_clear_bit

"sbitmap_batch_clear" should be "sbitmap_deferred_clear"

Acked-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>