bcachefs: Cleanup redundant snapshot nodes

After deleteing snapshots, we may be left with a snapshot tree where
some nodes only have one child, and we have a linear chain.

Interior snapshot nodes are never used directly (i.e. they never have
subvolumes that point to them), they are only referered to by child
snapshot nodes - hence, they are redundant.

The existing code talks about redundant snapshot nodes as forming and
equivalence class; i.e. nodes for which snapshot_t->equiv is equal. In a
given equivalence class, we only ever need a single key at a given
position - i.e. multiple versions with different snapshot fields are
redundant.

The existing snapshot cleanup code deletes these redundant keys, but not
redundant nodes. It turns out this is buggy, because we assume that
after snapshot deletion finishes we should only have a single key per
equivalence class, but the btree update path doesn't preserve this -
overwriting keys in old snapshots doesn't check for the equivalence
class being equal, and thus we can end up with duplicate keys in the
same equivalence class and fsck complaining about snapshot deletion not
having run correctly.

The equivalence class notion has been leaking out of the core snapshots
code and into too much other code, i.e. fsck, so this patch takes a
different approach: snapshot deletion now moves keys to the node in an
equivalence class being kept (the leafiest node) and then deletes the
redundant nodes in the equivalance class.

Some work has to be done to correctly delete interior snapshot nodes;
snapshot node depth and skiplist fields for descendent nodes have to be
fixed.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs_format.h

@@ -1150,6 +1150,7 @@ struct bch_snapshot {
 	__le32			parent;
 	__le32			children[2];
 	__le32			subvol;
+	/* corresponds to a bch_snapshot_tree in BTREE_ID_snapshot_trees */
 	__le32			tree;
 	__le32			depth;
 	__le32			skip[3];