xen/pvcalls: bound backend response req_id before indexing rsp[]

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

xen/pvcalls: bound backend response req_id before indexing rsp[]

pvcalls_front_event_handler() takes req_id directly from the
backend-supplied ring response and uses it to index the fixed-size
bedata->rsp[] array for a memcpy() and a store, with no range check. A
malicious or buggy backend can set req_id past PVCALLS_NR_RSP_PER_RING
and drive an out-of-bounds write past the bedata allocation.

req_id was also declared int while the wire field rsp->req_id is u32, so
a range check on the signed value alone is insufficient: a backend
req_id of 0xffffffff becomes -1, passes a >= PVCALLS_NR_RSP_PER_RING
test and indexes bedata->rsp[-1]. Declare req_id as u32 so a single
bound covers both ends.

A backend that sends an out-of-range req_id has violated the wire
protocol, so rather than silently dropping the response, log once and
stop trusting the backend: set bedata->disabled. The event handler then
ignores further responses, and the request paths that wait for a
response return -EIO instead of blocking forever. This mirrors the
fatal-error handling xen-netback uses (xenvif_fatal_tx_err()).

The pvcalls frontend currently trusts its backend, so this is not a
classic-Xen security issue, but it matters for hardening PV frontends
against malicious backends (confidential and disaggregated deployments).