mshv: Order pt_vp_array publish against irqfd assertion path

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

mshv: Order pt_vp_array publish against irqfd assertion path

mshv_partition_ioctl_create_vp() initialises a VP struct (allocations,
mutex_init, init_waitqueue_head, page mappings) and then publishes the
pointer into partition->pt_vp_array. Several ISR paths read this array
locklessly: the intercept ISR, the two scheduler ISRs, and
mshv_try_assert_irq_fast() on the irqfd fast path.

Of these, only mshv_try_assert_irq_fast() can structurally race the
publish. It runs from an eventfd waker without holding pt_mutex, and
MSHV_IRQFD does not require the target lapic_apic_id (== vp_index) to
refer to an existing VP at registration time. A user can therefore
register an irqfd targeting a yet-to-be-created VP, then trigger
mshv_try_assert_irq_fast() concurrently with MSHV_CREATE_VP for the
same index. On weakly-ordered architectures the reader can observe a
non-NULL pointer in pt_vp_array before the initialising stores to the
VP struct become visible, leading to use of partially-initialised
fields (e.g. vp_register_page).

The other ISR readers cannot reach this race: the hypervisor will not
generate intercept or scheduler messages for a VP that has never been
told to run, and the user can only call MSHV_RUN_VP on the VP fd
returned by MSHV_CREATE_VP, which by construction is returned after
the publish. Leave those readers as plain loads.

Use smp_store_release() in mshv_partition_ioctl_create_vp() to publish
the pointer, and pair it with smp_load_acquire() in
mshv_try_assert_irq_fast(). On x86 these compile to plain accesses
under TSO; on ARM64 they emit one-instruction acquire/release barriers,
acceptable on this fast path.

The destroy-side path (destroy_partition() clearing pt_vp_array[i] to
NULL after kfree(vp)) has a separate ordering and lifetime concern
that is out of scope here.