bcachefs: Fix incorrectly freeing btree_path in alloc path

Clearing path->preserve means the path will be dropping in
bch2_trans_begin() - but on transaction restart, we're likely to need
that path again.

This fixes a livelock in the allocation path.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_iter.h

@@ -335,6 +335,7 @@ void bch2_trans_copy_iter(struct btree_iter *, struct btree_iter *);
 
 static inline void set_btree_iter_dontneed(struct btree_iter *iter)
 {
+	if (!iter->trans->restarted)
 		iter->path->preserve = false;
 }