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 <[e-mail]>
Reported-by: Dunphy, Bill <[e-mail]>
Signed-off-by: Ryusuke Konishi <[e-mail]>
Tested-by: Bruno Prémont <[e-mail]>
Cc: stable <[e-mail]>

fs/nilfs2/btnode.c

···
 				       "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);