bcachefs: Move btree node merging to before transaction commit

bcachefs: Move btree node merging to before transaction commit

Currently, BTREE_INSERT_NOUNLOCK makes it hard to ensure btree node
merging happens reliably - since btree node merging happens after
transaction commit, we can't drop btree locks and block when starting
the btree update.

This patch moves it to before transaction commit - and failure to do a
merge that we wanted to do just restarts the transaction.

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