bcachefs: Fix split_race livelock

bch2_btree_update_start() calculates which nodes are going to have to be
split/rewritten, so that we know how many nodes to reserve and how deep
in the tree we have to take locks.

But btree node merges require inserting two keys into the parent node,
not just splits.

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