migration: Remove RAMState.f references in compression code

Removing referencing to RAMState.f in compress_page_with_multi_thread() and
flush_compressed_data().

Compression code by default isn't compatible with having >1 channels (or it
won't currently know which channel to flush the compressed data), so to
make it simple we always flush on the default to_dst_file port until
someone wants to add >1 ports support, as rs->f right now can really
change (after postcopy preempt is introduced).

There should be no functional change at all after patch applied, since as
long as rs->f referenced in compression code, it must be to_dst_file.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -1489,6 +1489,7 @@ static bool save_page_use_compression(RAMState *rs);
 
 static void flush_compressed_data(RAMState *rs)
 {
+    MigrationState *ms = migrate_get_current();
     int idx, len, thread_count;
 
     if (!save_page_use_compression(rs)) {
@@ -1507,7 +1508,7 @@ static void flush_compressed_data(RAMState *rs)
     for (idx = 0; idx < thread_count; idx++) {
         qemu_mutex_lock(&comp_param[idx].mutex);
         if (!comp_param[idx].quit) {
-            len = qemu_put_qemu_file(rs->f, comp_param[idx].file);
+            len = qemu_put_qemu_file(ms->to_dst_file, comp_param[idx].file);
             /*
              * it's safe to fetch zero_page without holding comp_done_lock
              * as there is no further request submitted to the thread,
@@ -1526,19 +1527,20 @@ static inline void set_compress_params(CompressParam *param, RAMBlock *block,
     param->offset = offset;
 }
 
-static int compress_page_with_multi_thread(RAMState *rs, RAMBlock *block,
-                                           ram_addr_t offset)
+static int compress_page_with_multi_thread(RAMBlock *block, ram_addr_t offset)
 {
     int idx, thread_count, bytes_xmit = -1, pages = -1;
     bool wait = migrate_compress_wait_thread();
+    MigrationState *ms = migrate_get_current();
 
     thread_count = migrate_compress_threads();
     qemu_mutex_lock(&comp_done_lock);
 retry:
     for (idx = 0; idx < thread_count; idx++) {
         if (comp_param[idx].done) {
             comp_param[idx].done = false;
-            bytes_xmit = qemu_put_qemu_file(rs->f, comp_param[idx].file);
+            bytes_xmit = qemu_put_qemu_file(ms->to_dst_file,
+                                            comp_param[idx].file);
             qemu_mutex_lock(&comp_param[idx].mutex);
             set_compress_params(&comp_param[idx], block, offset);
             qemu_cond_signal(&comp_param[idx].cond);
@@ -2291,7 +2293,7 @@ static bool save_compress_page(RAMState *rs, RAMBlock *block, ram_addr_t offset)
         return false;
     }
 
-    if (compress_page_with_multi_thread(rs, block, offset) > 0) {
+    if (compress_page_with_multi_thread(block, offset) > 0) {
         return true;
     }