Revert "block: complete all IOs before .bdrv_truncate"

brdv_truncate() is also called from readv/writev commands on self-
growing file based storage. this will result in requests waiting
for theirselves to complete.

This reverts commit 9a665b2b

.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2487,10 +2487,6 @@ int bdrv_truncate(BlockDriverState *bs, int64_t offset)
         return -EACCES;
     if (bdrv_in_use(bs))
         return -EBUSY;
-
-    /* There better not be any in-flight IOs when we truncate the device. */
-    bdrv_drain_all();
-
     ret = drv->bdrv_truncate(bs, offset);
     if (ret == 0) {
         ret = refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS);