AMD/IOMMU: Process softirqs while building dom0 iommu mappings

AMD/IOMMU: Process softirqs while building dom0 iommu mappings

Recent changes which have made their way into xen-4.2 stable have pushed the
runtime of construct_dom0() over 5 seconds, which has caused regressions in
XenServer testing because of our 5 second watchdog.

The root cause is that amd_iommu_dom0_init() does not process softirqs and in
particular the nmi_timer which causes the watchdog to decide that no useful
progress is being made.

This patch adds periodic calls to process_pending_softirqs() at the same
interval as the Intel variant of this function.  The server which was failing
with the watchdog test now boots reliably with a timeout of 1 second.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>