Btrfs: Wait for async bio submissions to make some progress at queue time

Before, the btrfs bdi congestion function was used to test for too many
async bios.  This keeps that check to throttle pdflush, but also
adds a check while queuing bios.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/transaction.c

@@ -322,8 +322,6 @@ int btrfs_write_and_wait_transaction(struct btrfs_trans_handle *trans,
 		if (ret)
 			break;
 		while(start <= end) {
-			if (btrfs_congested_async(root->fs_info, 0))
-				congestion_wait(WRITE, HZ/10);
 			cond_resched();
 
 			index = start >> PAGE_CACHE_SHIFT;