btrfs: implement ref_tracker for delayed_nodes

Add ref_tracker infrastructure for struct btrfs_delayed_node.

It is a response to the largest btrfs related crash in our fleet.  We're
seeing soft lockups in btrfs_kill_all_delayed_nodes() that seem to be a
result of delayed_nodes not being released properly.

A ref_tracker object is allocated on reference count increases and freed
on reference count decreases. The ref_tracker object stores a stack
trace of where it is allocated. The ref_tracker_dir object is embedded
in btrfs_delayed_node and keeps track of all current and some old/freed
ref_tracker objects. When a leak is detected we can print the stack
traces for all ref_trackers that have not yet been freed.

Here is a common example of taking a reference to a delayed_node and
freeing it with ref_tracker.

    struct btrfs_ref_tracker tracker;
    struct btrfs_delayed_node *node;

    node = btrfs_get_delayed_node(inode, &tracker);
    // use delayed_node...
    btrfs_release_delayed_node(node, &tracker);

There are two special cases where the delayed_node reference is "long
lived", meaning that the thread that takes the reference and the thread
that releases the reference are different. The 'inode_cache_tracker'
tracks the delayed_node stored in btrfs_inode. The 'node_list_tracker'
tracks the delayed_node stored in the btrfs_delayed_root
node_list/prepare_list. These trackers are embedded in the
btrfs_delayed_node.

btrfs_ref_tracker and btrfs_ref_tracker_dir are wrappers that either
compile to the corresponding ref_tracker structs or empty structs
depending on CONFIG_BTRFS_DEBUG. There are also btrfs wrappers for
the ref_tracker API.

Signed-off-by: Leo Martins <loemra.dev@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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