bcachefs: Write new btree nodes after parent update

bcachefs: Write new btree nodes after parent update

In order to avoid locking all btree nodes up to the root for btree node
splits, we're going to have to introduce a new error path into
bch2_btree_insert_node(); this mean we can't have done any writes or
modified global state before that point.

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