vfio/pds: Only use a single SGL for both seq and ack

vfio/pds: Only use a single SGL for both seq and ack

Since the seq/ack operations never happen in parallel there
is no need for multiple scatter gather lists per region.
The current implementation is wasting memory. Fix this by
only using a single scatter-gather list for both the seq
and ack operations.

Signed-off-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Link: https://lore.kernel.org/r/20231117001207.2793-3-brett.creeley@amd.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>