Btrfs: Throttle tuning

This avoids waiting for transactions with pages locked by breaking out
the code to wait for the current transaction to close into a function
called by btrfs_throttle.

It also lowers the limits for where we start throttling.

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

fs/btrfs/extent-tree.c

@@ -2526,6 +2526,9 @@ static int noinline walk_down_tree(struct btrfs_trans_handle *trans,
 				*level = 0;
 				break;
 			}
+			if (printk_ratelimit())
+				printk("leaf ref miss for bytenr %llu\n",
+				       (unsigned long long)bytenr);
 		}
 		next = btrfs_find_tree_block(root, bytenr, blocksize);
 		if (!next || !btrfs_buffer_uptodate(next, ptr_gen)) {