drivers/block/xen-blkback/common.h: Fix spelling typo in comment

Fix spelling typo in comment.

Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: liyouhong <liyouhong@kylinos.cn>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20231226095701.172080-1-liyouhong@kylinos.cn


Signed-off-by: Jens Axboe <axboe@kernel.dk>