f2fs: fix missing up_read

This patch fixes missing up_read call.

Fixes: c9b60788 ("f2fs: fix to do sanity check with block address in main area")
Cc: <stable@vger.kernel.org> # 4.19+
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>