migration/ram.c: Remove the qemu_mutex_lock in colo_flush_ram_cache.

The code to acquire bitmap_mutex is added in the commit of
"63268c49". There is no
need to acquire bitmap_mutex in colo_flush_ram_cache(). This
is because the colo_flush_ram_cache only be called on the COLO
secondary VM, which is the destination side.
On the COLO secondary VM, only the COLO thread will touch
the bitmap of ram cache.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -3918,7 +3918,6 @@ void colo_flush_ram_cache(void)
     unsigned long offset = 0;
 
     memory_global_dirty_log_sync();
-    qemu_mutex_lock(&ram_state->bitmap_mutex);
     WITH_RCU_READ_LOCK_GUARD() {
         RAMBLOCK_FOREACH_NOT_IGNORED(block) {
             ramblock_sync_dirty_bitmap(ram_state, block);
@@ -3954,7 +3953,6 @@ void colo_flush_ram_cache(void)
         }
     }
     trace_colo_flush_ram_cache_end();
-    qemu_mutex_unlock(&ram_state->bitmap_mutex);
 }
 
 /**