KVM: arm64: Tag host-VA hypercall parameters __kern

The nVHE hypervisor takes host virtual addresses as hypercall arguments
and translates each with kern_hyp_va() before use. Nothing marks them as
host-owned, so dereferencing one untranslated at EL2 - a recurring bug
class - is invisible to the compiler.

Add a __kern sparse address space, active only for EL2 code, and tag the
host-VA parameters in the hypercall declarations. kern_hyp_va_host() is
the only sanctioned unwrap: it translates the address, preserves the
pointee type (stripped of qualifiers, as with the percpu accessors) and
drops the tag with a __force cast, so an untranslated host VA fails
sparse. The tag flows from the shared declaration into the generated
handler and on into the donated-memory and tracing-descriptor helpers,
so a handler cannot extract a host VA without it. Host code sees plain
pointers, and the tag is checker-only: no code is generated.

container_of() casts through void * and drops the address space, so
__get_host_hyp_vcpus() now takes an already translated vCPU and its
callers unwrap. Translating a vgic_v3_cpu_if before taking its container
is equivalent, since va_mask spans every bit in which two linear-map
addresses differ.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>