bcachefs: Split out journal pins by btree level

This lets us flush the journal to go read-only more effectively.

Flushing the journal and going read-only requires halting mutually
recursive processes, which strictly speaking are not guaranteed to
terminate.

Flushing btree node journal pins will kick off a btree node write, and
btree node writes on completion must do another btree update to the
parent node to update the 'sectors_written' field for that node's key.

If the parent node is full and requires a split or compaction, that's
going to generate a whole bunch of additional btree updates - alloc
info, LRU btree, and more - which then have to be flushed, and the cycle
repeats.

This process will terminate much more effectively if we tweak journal
reclaim to flush btree updates leaf to root: i.e., don't flush updates
for a given btree node (kicking off a write, and consuming space within
that node up to the next block boundary) if there might still be
unflushed updates in child nodes.

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