vfs: Fix endless loop in bdev_read and bdev_write

If iov->iov_len == 0, it will loop forever.
uiomove() will take care of the zero iov len case.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>