btrfs: remove unnecessary leaf free space checks when pushing items

When trying to push items from a leaf into its left and right neighbours,
we lock the left or right leaf, check if it has the required minimum free
space, COW the leaf and then check again if it has the minimum required
free space. This second check is pointless:

1) Most and foremost because it's not needed. We have a write lock on the
   leaf and on its parent node, so no one can come in and change either
   the pre-COW or post-COW version of the leaf for the whole duration of
   the push_leaf_left() and push_leaf_right() calls;

2) The call to btrfs_leaf_free_space() is not trivial, it has a fair
   amount of arithmetic operations and access to fields in the leaf's
   header and items, so it's not very cheap.

So remove the duplicated free space checks.

This change if part of a patchset that is comprised of the following
patches:

  1/6 btrfs: remove unnecessary leaf free space checks when pushing items
  2/6 btrfs: avoid unnecessary COW of leaves when deleting items from a leaf
  3/6 btrfs: avoid unnecessary computation when deleting items from a leaf
  4/6 btrfs: remove constraint on number of visited leaves when replacing extents
  5/6 btrfs: remove useless path release in the fast fsync path
  6/6 btrfs: prepare extents to be logged before locking a log tree path

The last patch in the series has some performance test result in its
changelog.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -2990,16 +2990,11 @@ static int push_leaf_right(struct btrfs_trans_handle *trans, struct btrfs_root
 	if (free_space < data_size)
 		goto out_unlock;
 
-	/* cow and double check */
 	ret = btrfs_cow_block(trans, root, right, upper,
 			      slot + 1, &right, BTRFS_NESTING_RIGHT_COW);
 	if (ret)
 		goto out_unlock;
 
-	free_space = btrfs_leaf_free_space(right);
-	if (free_space < data_size)
-		goto out_unlock;
-
 	left_nritems = btrfs_header_nritems(left);
 	if (left_nritems == 0)
 		goto out_unlock;
@@ -3224,7 +3219,6 @@ static int push_leaf_left(struct btrfs_trans_handle *trans, struct btrfs_root
 		goto out;
 	}
 
-	/* cow and double check */
 	ret = btrfs_cow_block(trans, root, left,
 			      path->nodes[1], slot - 1, &left,
 			      BTRFS_NESTING_LEFT_COW);
@@ -3235,12 +3229,6 @@ static int push_leaf_left(struct btrfs_trans_handle *trans, struct btrfs_root
 		goto out;
 	}
 
-	free_space = btrfs_leaf_free_space(left);
-	if (free_space < data_size) {
-		ret = 1;
-		goto out;
-	}
-
 	if (check_sibling_keys(left, right)) {
 		ret = -EUCLEAN;
 		goto out;