Service interruption via rounded-down EOF validation

Improper EOF validation in Linux kernel branches 6.6, 6.8, and 6.9 allows local users to interrupt service via an unaligned block write. The blkdev_iomap_begin function in block/fops.c rounds the requested position down to the logical block size, then checks the rounded iomap offset against inode size instead of checking the raw position, producing a zero-length mapping for an unaligned position at EOF. Low-privileged local access and the ability to issue the crafted block-device write are required; the reported consequence is availability loss without confidentiality or integrity impact.