bcachefs: Increase max size for btree_trans bump allocator

With backpointers, alloc keys have gotten bigger, so we're needing more
memory here.

We're probably going to need to go with something more sophisticated
than a bump allocator, but - let's see if we can avoid doing that just
yet.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_types.h

@@ -374,7 +374,7 @@ struct btree_trans_commit_hook {
 	struct btree_trans_commit_hook	*next;
 };
 
-#define BTREE_TRANS_MEM_MAX	(1U << 14)
+#define BTREE_TRANS_MEM_MAX	(1U << 16)
 
 struct btree_trans {
 	struct bch_fs		*c;