migration/postcopy: reduce one operation to calculate fixup_start_addr

Use the same way for run_end to calculate run_start, which saves one
operation.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190710050814.31344-2-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -2973,10 +2973,12 @@ static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
         host_offset = run_start % host_ratio;
         if (host_offset) {
             do_fixup = true;
-            run_start -= host_offset;
-            fixup_start_addr = run_start;
-            /* For the next pass */
-            run_start = run_start + host_ratio;
+            fixup_start_addr = run_start - host_offset;
+            /*
+             * This host page has gone, the next loop iteration starts
+             * from after the fixup
+             */
+            run_start = fixup_start_addr + host_ratio;
         } else {
             /* Find the end of this run */
             unsigned long run_end;