block: clean up the check in blkdev_iomap_begin()

block: clean up the check in blkdev_iomap_begin()

It is odd to check the offset amidst a series of assignments. Moving this
check to the beginning of the function makes the code look better.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240625115517.1472120-1-linan666@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>