f2fs: remove repeated f2fs_bug_on

f2fs: remove repeated f2fs_bug_on which has already existed
      in function invalidate_blocks.

Signed-off-by: Zhikang Zhang <zhangzhikang1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>