block: Use bdrv_drain to replace uncessary bdrv_drain_all

There callers work on a single BlockDriverState subtree, where using
bdrv_drain() is more accurate.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block.c

diff --git a/block.c b/block.c
index 42eb8e3..5e80336 100644
--- a/block.c
+++ b/block.c
@@ -1841,9 +1841,9 @@
     if (bs->job) {
         block_job_cancel_sync(bs->job);
     }
-    bdrv_drain_all(); /* complete I/O */
+    bdrv_drain(bs); /* complete I/O */
     bdrv_flush(bs);
-    bdrv_drain_all(); /* in case flush left pending I/O */
+    bdrv_drain(bs); /* in case flush left pending I/O */
     notifier_list_notify(&bs->close_notifiers, bs);
 
     if (bs->drv) {
@@ -3906,7 +3906,7 @@
 
 void bdrv_set_aio_context(BlockDriverState *bs, AioContext *new_context)
 {
-    bdrv_drain_all(); /* ensure there are no in-flight requests */
+    bdrv_drain(bs); /* ensure there are no in-flight requests */
 
     bdrv_detach_aio_context(bs);
 

block/snapshot.c

diff --git a/block/snapshot.c b/block/snapshot.c
index 19395ae..49e143e 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -239,7 +239,7 @@
     }
 
     /* drain all pending i/o before deleting snapshot */
-    bdrv_drain_all();
+    bdrv_drain(bs);
 
     if (drv->bdrv_snapshot_delete) {
         return drv->bdrv_snapshot_delete(bs, snapshot_id, name, errp);

migration/block.c

diff --git a/migration/block.c b/migration/block.c
index ddb59cc..ed865ed 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -457,7 +457,7 @@
         blk_mig_lock();
         if (bmds_aio_inflight(bmds, sector)) {
             blk_mig_unlock();
-            bdrv_drain_all();
+            bdrv_drain(bmds->bs);
         } else {
             blk_mig_unlock();
         }