ext4: show test_dummy_encryption mount option in /proc/mounts

When in effect, add "test_dummy_encryption" to _ext4_show_options() so
that it is shown in /proc/mounts and other relevant procfs files.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/super.c

@@ -2145,6 +2145,8 @@ static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
 		SEQ_OPTS_PRINT("max_dir_size_kb=%u", sbi->s_max_dir_size_kb);
 	if (test_opt(sb, DATA_ERR_ABORT))
 		SEQ_OPTS_PUTS("data_err=abort");
+	if (DUMMY_ENCRYPTION_ENABLED(sbi))
+		SEQ_OPTS_PUTS("test_dummy_encryption");
 
 	ext4_show_quota_options(seq, sb);
 	return 0;