block: refine the EOF check in blkdev_iomap_begin

In the Linux kernel, the following vulnerability has been resolved:

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.