bcache: fix potential deadlock problem in btree_gc_coalesce

hulk inclusion
category: bugfix
bugzilla: 13690
CVE: CVE-2020-12771

---------------------------

coccicheck reports:
  drivers/md//bcache/btree.c:1538:1-7: preceding lock on line 1417

btree_gc_coalesce func is designed to coalesce two adjacent nodes in
new_nodes[GC_MERGE_NODES] and finally release one node. All nodes`write_lock,
new_nodes[i]->write_lock, are holded before coalescing adjacent nodes,
and them will be released after coalescing successfully.

However, if the coalescing process fails, such as no enough space of new_nodes[1]
to fit all of the remaining keys in new_nodes[0] and realloc keylist failed, we
will goto to out_nocoalesce tag directly without releasing new_nodes[i]->write_lock.
Then, a deadlock will occur after calling btree_node_free to free new_nodes[i],
which also try to acquire new_nodes[i]->write_lock.

Here, we add a new tag 'out_unlock_nocoalesce' before out_nocoalesce tag to release
new_nodes[i]->write_lock when coalescing process fails.

Fixes: 2a285686 ("bcache: btree locking rework")
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>