btrfs-progs: do not dump leaf if the path is released inside __free_extent()

[BUG]
There is a bug report that btrfs-convert crashed during converting an
ext4 image which is almost full.

[CAUSE]
Just before the crash, btrfs-convert is committing the current
transaction but failed to locate the backref inside __free_extent().

Then it went through the error handling path, which printed an error
message and tried to dump the leaf.

But in this particular case, the error code is not -ENOENT, thus the
path is already released, resulting path->nodes[0] to be NULL, and
btrfs_print_leaf() triggers a NULL pointer dereference.

[FIX]
The kernel version of btrfs_free_extent() is only dumping the tree for
-ENOENT error code. And patch "btrfs: refactor the error handling of
__btrfs_free_extent()" was submitted to make abort_and_dump() to only
dump the leaf if the path is not released.

So follow the same kernel patch, by only dumping the leaf if the path is
not released.

Issue: #1064
Pull-request: #1065
Signed-off-by: Qu Wenruo <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: adam900710

kernel-shared/extent-tree.c

@@ -2058,8 +2058,10 @@ static int __free_extent(struct btrfs_trans_handle *trans,
 		       (unsigned long long)root_objectid,
 		       (unsigned long long)owner_objectid,
 		       (unsigned long long)owner_offset);
-		printf("path->slots[0]: %d path->nodes[0]:\n", path->slots[0]);
-		btrfs_print_leaf(path->nodes[0]);
+		if (path->nodes[0]) {
+			printf("path->slots[0]: %d path->nodes[0]:\n", path->slots[0]);
+			btrfs_print_leaf(path->nodes[0]);
+		}
 		ret = -EIO;
 		goto fail;
 	}