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: 487c607d ("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>