AMD/IOMMU: correct IRTE updating

AMD/IOMMU: correct IRTE updating

Flushing didn't get done along the lines of what the specification says.
Mark entries to be updated as not remapped (which will result in
interrupt requests to get target aborted, but the interrupts should be
masked anyway at that point in time), issue the flush, and only then
write the new entry.

In update_intremap_entry_from_msi_msg() also fold the duplicate initial
lock determination and acquire into just a single instance.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Brian Woods <brian.woods@amd.com>