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