bcachefs: Ensure btree node cache is not more than half dirty

Tweak journal reclaim to ensure the btree node cache isn't more
than half dirty so that memory reclaim can always make progress - the
same as we do for the btree key cache.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/journal_reclaim.c

@@ -640,6 +640,9 @@ static int __bch2_journal_reclaim(struct journal *j, bool direct, bool kicked)
 		if (fifo_free(&j->pin) <= 32)
 			min_nr = 1;
 
+		if (atomic_read(&c->btree_cache.dirty) * 2 > c->btree_cache.used)
+			min_nr = 1;
+
 		min_key_cache = min(bch2_nr_btree_keys_need_flush(c), (size_t) 128);
 
 		trace_and_count(c, journal_reclaim_start, c,