f2fs: make assignment of t->dentry_bitmap more readable

In make_dentry_ptr_block, it is confused with "&" for t->dentry_bitmap
but without "&" for t->dentry, so delete "&" to make code more readable.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>