iomap: simplify iomap_dio_bio_iter

iomap: simplify iomap_dio_bio_iter

Use iov_iter_count to check if we need to continue as that just reads
a field in the iov_iter, and only use bio_iov_vecs_to_alloc to calculate
the actual number of vectors to allocate for the bio.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>