block: refine the EOF check in blkdev_iomap_begin

blkdev_iomap_begin rounds down the offset to the logical block size
before stashing it in iomap->offset and checking that it still is
inside the inode size.

Check the i_size check to the raw pos value so that we don't try a
zero size write if iter->pos is unaligned.

Fixes: 487c607 ("block: use iomap for writes to block devices")
Reported-by: syzbot+0a3683a0a6fecf909244@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: syzbot+0a3683a0a6fecf909244@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20240503081042.2078062-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Christoph Hellwig

block/fops.c

@@ -385,7 +385,7 @@ static int blkdev_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
 
 	iomap->bdev = bdev;
 	iomap->offset = ALIGN_DOWN(offset, bdev_logical_block_size(bdev));
-	if (iomap->offset >= isize)
+	if (offset >= isize)
 		return -EIO;
 	iomap->type = IOMAP_MAPPED;
 	iomap->addr = iomap->offset;