btrfs: Adjust commit-transaction condition to avoid NO_SPACE more

If we have any chance to make a successful write, we should not give up.

This patch adjust commit-transaction condition from:
  pinned >= wanted
to
  left + pinned >= wanted

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -3931,7 +3931,8 @@ alloc:
 		 * don't bother committing the transaction.
 		 */
 		if (percpu_counter_compare(&data_sinfo->total_bytes_pinned,
-					   bytes) < 0)
+					   used + bytes -
+					   data_sinfo->total_bytes) < 0)
 			have_pinned_space = 0;
 		spin_unlock(&data_sinfo->lock);