bcachefs: Fix allocate -> self healing path

When we go to allocate and find taht a bucket in the freespace btree is
actually allocated, we're supposed to return nonzero to tell the
allocator to skip it.

This fixes an emergency read only due to a bucket/ptr gen mismatch - we
also don't return the correct bucket gen when this happens.

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

fs/bcachefs/alloc_background.c

@@ -1475,6 +1475,8 @@ int bch2_check_discard_freespace_key(struct btree_trans *trans, struct btree_ite
 		w->c = c;
 		w->pos = BBPOS(iter->btree_id, iter->pos);
 		queue_work(c->write_ref_wq, &w->work);
+
+		ret = 1; /* don't allocate from this bucket */
 		goto out;
 	}
 }