AMD/IOMMU: re-work locking around sending of commands

AMD/IOMMU: re-work locking around sending of commands

It appears unhelpful to me for flush_command_buffer() to block all
progress elsewhere for the given IOMMU by holding its lock while waiting
for command completion. There's no real need for callers of that
function or of send_iommu_command() to hold the lock. Contain all
command sending related locking to the latter function.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>