xen/public: add comment to struct xen_mem_acquire_resource

Commit 7c7f7e8fba changed xen/include/public/memory.h in an incompatible
way. Unfortunately the changed parts were already in use in the Linux
kernel, so an update of the header in the kernel would result in a build
breakage.

As the change of above commit was in a section originally meant to be not
stable, it was the usage in the kernel which was wrong.

Add a comment to the modified struct for not reusing the now removed bit,
in order to avoid kernels using it stumbling over a possible new meaning
of the bit.

In case the kernel is updating to a new version of the header, the wrong
use case must be removed first.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/include/public/memory.h

@@ -662,6 +662,13 @@ struct xen_mem_acquire_resource {
      * two calls.
      */
     uint32_t nr_frames;
+    /*
+     * Padding field, must be zero on input.
+     * In a previous version this was an output field with the lowest bit
+     * named XENMEM_rsrc_acq_caller_owned. Future versions of this interface
+     * will not reuse this bit as an output with the field being zero on
+     * input.
+     */
     uint32_t pad;
     /*
      * IN - the index of the initial frame to be mapped. This parameter