vringh: fix __vringh_iov() when riov and wiov are different

vringh: fix __vringh_iov() when riov and wiov are different

If riov and wiov are both defined and they point to different
objects, only riov is initialized. If the wiov is not initialized
by the caller, the function fails returning -EINVAL and printing
"Readable desc 0x... after writable" error message.

This issue happens when descriptors have both readable and writable
buffers (eg. virtio-blk devices has virtio_blk_outhdr in the readable
buffer and status as last byte of writable buffer) and we call
__vringh_iov() to get both type of buffers in two different iovecs.

Let's replace the 'else if' clause with 'if' to initialize both
riov and wiov if they are not NULL.

As checkpatch pointed out, we also avoid crashing the kernel
when riov and wiov are both NULL, replacing BUG() with WARN_ON()
and returning -EINVAL.

Fixes: f87d0fbb5798 ("vringh: host-side implementation of virtio rings.")
Cc: stable@vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201008204256.162292-1-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>