nilfs2: fix dirty page accounting leak causing hang at write

Bruno Prémont and Dunphy, Bill noticed me that NILFS will certainly
hang on ARM-based targets.

I found this was caused by an underflow of dirty pages counter.  A
b-tree cache routine was marking page dirty without adjusting page
account information.

This fixes the dirty page accounting leak and resolves the hang on
arm-based targets.
Reported-by: Bruno Prémont <bonbons@linux-vserver.org>
Reported-by: Dunphy, Bill <WDunphy@tandbergdata.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Tested-by: Bruno Prémont <bonbons@linux-vserver.org>
Cc: stable <stable@kernel.org>

fs/nilfs2/btnode.c

@@ -276,8 +276,7 @@ void nilfs_btnode_commit_change_key(struct address_space *btnc,
 				       "invalid oldkey %lld (newkey=%lld)",
 				       (unsigned long long)oldkey,
 				       (unsigned long long)newkey);
-		if (!test_set_buffer_dirty(obh) && TestSetPageDirty(opage))
-			BUG();
+		nilfs_btnode_mark_dirty(obh);
 
 		spin_lock_irq(&btnc->tree_lock);
 		radix_tree_delete(&btnc->page_tree, oldkey);