f2fs: fix to truncate dentry pages in the error case

When a new directory is allocated, if an error is occurred, we should truncate
preallocated dentry pages too.

This bug was reported by Andrey Tsyvarev after a while as follows.

mkdir()->
 f2fs_add_link()->
  init_inode_metadata()->
    f2fs_init_acl()->
      f2fs_get_acl()->
        f2fs_getxattr()->
          read_all_xattrs() fails.

Also there was a BUG_ON triggered after the fault in
mkdir()->
 f2fs_add_link()->
   init_inode_metadata()->
    remove_inode_page() ->
      f2fs_bug_on(inode->i_blocks != 0 && inode->i_blocks != 1);

But, previous patch wasn't perfect to resolve that bug, so the following bug
report was also submitted.

kernel BUG at fs/f2fs/inode.c:274!
Call Trace:
 [<ffffffff811fde03>] evict+0xa3/0x1a0
 [<ffffffff811fe615>] iput+0xf5/0x180
 [<ffffffffa01c7f63>] f2fs_mkdir+0xf3/0x150 [f2fs]
 [<ffffffff811f2a77>] vfs_mkdir+0xb7/0x160
 [<ffffffff811f36bf>] SyS_mkdir+0x5f/0xc0
 [<ffffffff81680769>] system_call_fastpath+0x16/0x1b

Finally, this patch resolves all the issues like below.

If an error is occurred after make_empty_dir(),
 1. truncate_inode_pages()
   The make_bad_inode() prior to iput() will change i_mode to S_IFREG, which
   means that f2fs will not decrement fi->dirty_dents during f2fs_evict_inode.
   But, by calling it here, we can do that.

 2. truncate_blocks()
   Preallocated dentry pages are trucated here to sync i_blocks.

 3. remove_dirty_dir_inode()
   Remove this directory inode from the list.

Reported-and-Tested-by: Andrey Tsyvarev <tsyvarev@ispras.ru>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>