migration/ram.c: Fix codes conflict about bitmap_mutex

I found upstream codes conflict with COLO and lead to crash,
and I located to this patch:

commit 386a907
Author: Wei Wang <wei.w.wang@intel.com>
Date:   Tue Dec 11 16:24:49 2018 +0800

migration: use bitmap_mutex in migration_bitmap_clear_dirty

My colleague Wei's patch add bitmap_mutex in migration_bitmap_clear_dirty,
but COLO didn't initialize the bitmap_mutex. So we always get an error
when COLO start up. like that:
qemu-system-x86_64: util/qemu-thread-posix.c:64: qemu_mutex_lock_impl: Assertion `mutex->initialized' failed.

This patch add the bitmap_mutex initialize and destroy in COLO
lifecycle.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Message-Id: <20190329222951.28945-1-chen.zhang@intel.com>
Reviewed-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -3918,6 +3918,7 @@ int colo_init_ram_cache(void)
     }
     ram_state = g_new0(RAMState, 1);
     ram_state->migration_dirty_pages = 0;
+    qemu_mutex_init(&ram_state->bitmap_mutex);
     memory_global_dirty_log_start();
 
     return 0;
@@ -3956,6 +3957,7 @@ void colo_release_ram_cache(void)
     }
 
     rcu_read_unlock();
+    qemu_mutex_destroy(&ram_state->bitmap_mutex);
     g_free(ram_state);
     ram_state = NULL;
 }