bcachefs: All held locks must be in a btree path

bcachefs: All held locks must be in a btree path

With the new deadlock cycle detector, it's critical that all held locks
be marked in a btree_path, because that's what the cycle detector
traverses - any locks that aren't correctly marked will cause deadlocks.

This changes the btree_path to allocate some btree_paths for the new
nodes, since until the final update is done we otherwise don't have a
path referencing them.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>