vhost/vdpa: reject VRING_NUM larger than device max

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

vhost/vdpa: reject VRING_NUM larger than device max

vhost_vring_set_num() accepts any non-zero power-of-two queue size that
fits in 16 bits. vhost-vdpa then passes that value to set_vq_num()
without comparing it with get_vq_num_max().

A process with access to /dev/vhost-vdpa-* can therefore configure a
queue larger than the device advertises. With vdpa_sim, the worker can
walk descriptors beyond the mapped descriptor ring. KASAN reports a
16-byte out-of-bounds read, corresponding to one vring_desc, in the
vringh IOTLB path:

BUG: KASAN: out-of-bounds in _copy_from_iter
Read of size 16
copy_from_iotlb
copydesc_iotlb
vringh_getdesc_iotlb
vdpasim_net_work

Cache get_vq_num_max() immediately after reset. Some backends derive
it from writable queue-size state, so querying it after SET_NUM may
return the current size instead of the device capability. Invalidate
the cached value before reset so a failed reset leaves SET_NUM
disabled.

For VHOST_SET_VRING_NUM, copy the complete vring state once and use
the same index and size for validation, vq->num, and set_vq_num().
This ensures that validation and use operate on the same copied values.