Previously, the btree has always been self contained and internally
consistent on disk without anything from the journal - the journal just
contained pointers to the btree roots.

However, this meant that btree node split or compact operations - i.e.
anything that changes btree node topology and involves updates to
interior nodes - would require that interior btree node to be written
immediately, which means emitting a btree node write that's mostly empty
(using 4k of space on disk if the filesystemm blocksize is 4k to only
write perhaps ~100 bytes of new keys).

More importantly, this meant most btree node writes had to be FUA, and
consumer drives have a history of slow and/or buggy FUA support - other
filesystes have been bit by this.

This patch changes the interior btree update path to journal updates to
interior nodes, after the writes for the new btree nodes have completed.
Best of all, it turns out to simplify the interior node update path
somewhat.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>