bcachefs: Don't use bch2_btree_node_lock_write_nofail() in btree split path

It turns out - btree splits happen with the rest of the transaction
still locked, to avoid unnecessary restarts, which means using nofail
doesn't work here - we can deadlock.

Fortunately, we now have the ability to return errors here.

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