hv_netvsc: Allocate send/receive buffers using vmbus_alloc_buffer()

On CoCo VMs without confidential VMBus, the netvsc send and receive buffers
must be made host-visible by decrypting them. These buffers are vmalloc'ed,
but set_memory_decrypted()/encrypted() do not work on vmalloc'ed memory.
This use case is (so far) unique to netvsc, so solve it locally rather than
changing the set_memory() or allocation APIs.

Use vmbus_alloc_buffer() to allocate the send and receive buffers, which
will make them host-visible. Store the list of memory chunks in the
netvsc_device struct so they can be individually freed later. Use
vmbus_establish_gpadl_caller_decrypted() so there is no attempt to decrypt
the virtual address.

Appropriately free the buffers with vmbus_free_buffer(). Because vunmap()
and set_memory_encrypted() must run in process context, replace the
rcu_head/call_rcu() pair used to defer free_netvsc_device() with
rcu_work/queue_rcu_work(). This also fixes a small race condition where the
buffers may be accessed while being re-encrypted by moving the
re-encryption after the RCU grace period.

Signed-off-by: Kameron Carr <kameroncarr@linux.microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

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