passthrough/vtd: Don't DMA to the stack in queue_invalidate_wait()

passthrough/vtd: Don't DMA to the stack in queue_invalidate_wait()

DMA-ing to the stack is considered bad practice.  In this case, if a
timeout occurs because of a sluggish device which is processing the
request, the completion notification will corrupt the stack of a
subsequent deeper call tree.

Place the poll_slot in a percpu area and DMA to that instead.

Fix the declaration of saddr in struct qinval_entry, to avoid a shift by
two.  The requirement here is that the DMA address is dword aligned,
which is covered by poll_slot's type.

This change does not address other issues.  Correlating completions
after a timeout with their request is a more complicated change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <JBeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>