nilfs2: fix missing zero-fill initialization of btree node cache

This will fix file system corruption which infrequently happens after
mount.  The problem was reported from users with the title "[NILFS
users] Fail to mount NILFS." (Message-ID:
<200908211918.34720.yuri@itinteg.net>), and so forth.  I've also
experienced the corruption multiple times on kernel 2.6.30 and 2.6.31.

The problem turned out to be caused due to discordance between
mapping->nrpages of a btree node cache and the actual number of pages
hung on the cache; if the mapping->nrpages becomes zero even as it has
pages, truncate_inode_pages() returns without doing anything.  Usually
this is harmless except it may cause page leak, but garbage collection
fairly infrequently sees a stale page remained in the btree node cache
of DAT (i.e. disk address translation file of nilfs), and induces the
corruption.

I identified a missing initialization in btree node caches was the
root cause.  This corrects the bug.

I've tested this for kernel 2.6.30 and 2.6.31.
Reported-by: Yuri Chislov <yuri@itinteg.net>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: stable <stable@kernel.org>

fs/nilfs2/btnode.c

@@ -36,6 +36,7 @@
 
 void nilfs_btnode_cache_init_once(struct address_space *btnc)
 {
+	memset(btnc, 0, sizeof(*btnc));
 	INIT_RADIX_TREE(&btnc->page_tree, GFP_ATOMIC);
 	spin_lock_init(&btnc->tree_lock);
 	INIT_LIST_HEAD(&btnc->private_list);