HAMMER VFS: Add missing hammer_rel_volume() call in deadlock/retry case.

A late collision in hammer_get_buffer() could result in a ref count leak
on a hammer_volume structure.  Fix the leak.

sys/vfs/hammer/hammer_ondisk.c

@@ -639,8 +639,10 @@ hammer_get_buffer(hammer_mount_t hmp, hammer_off_t buf_offset,
 	 * Insert the buffer into the RB tree and handle late collisions.
 	 */
 	if (RB_INSERT(hammer_buf_rb_tree, &hmp->rb_bufs_root, buffer)) {
-		hammer_unref(&buffer->io.lock);
+		hammer_unref(&buffer->io.lock);	/* safety */
 		--hammer_count_buffers;
+		hammer_rel_volume(volume, 0);
+		buffer->io.volume = NULL;	/* safety */
 		kfree(buffer, hmp->m_misc);
 		goto again;
 	}