virtio_blk: set log2blksz correctly

'log2blksz' in blk_desc structure must always be initialized, otherwise
it will cause a lot of weird failures in file operations.

For example, fs_set_blk_dev[_with_part]() examines a block device against
every file system with its probe function. In particular, ext4 file
system's ext4_probe() will calls fs_devread() to fetch a super block.
If log2blksz is 0, the actual 'read' size, i.e. block_len >> log2blksz, is
much bigger than a buffer's size, and it can end up with memory corruption.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Fixes: f4802209 ("virtio: Add block driver support")
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>