f2fs: don't allow encrypted operations without keys

This patch fixes the renaming bug on encrypted filenames, which was pointed by

 (ext4: don't allow encrypted operations without keys)

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>